No longer define magit-section's hook variables as custom options

Instead define them as regular variables and add the default hook
functions using `add-hook'.

We need to do this so that we can distribute `magit-section' as a
standalone package, which cannot use function that are defined in
`magit's on its hooks--they might not be defined since it is now
possible to load `magit-section' without loading `magit'.

Documentation/magit.org

@@ -8,7 +8,7 @@
 #+TEXINFO_DIR_CATEGORY: Emacs
 #+TEXINFO_DIR_TITLE: Magit: (magit).
 #+TEXINFO_DIR_DESC: Using Git from Emacs with Magit.
-#+SUBTITLE: for version 2.90.1 (v2.90.1-880-gf63077242+1)
+#+SUBTITLE: for version 2.90.1 (v2.90.1-895-g1f79b3d77+1)
 
 #+TEXINFO_DEFFN: t
 #+OPTIONS: H:4 num:3 toc:2
@@ -25,7 +25,7 @@ directly from within Emacs.  While many fine Git clients exist, only
 Magit and Git itself deserve to be called porcelains.
 
 #+TEXINFO: @noindent
-This manual is for Magit version 2.90.1 (v2.90.1-880-gf63077242+1).
+This manual is for Magit version 2.90.1 (v2.90.1-895-g1f79b3d77+1).
 
 #+BEGIN_QUOTE
 Copyright (C) 2015-2020 Jonas Bernoulli <[email protected]>
@@ -921,6 +921,16 @@ the following commands.
 The above commands all call the hook ~magit-section-movement-hook~.
 Any of the functions listed below can be used as members of this hook.
 
+You might want to remove some of the functions that Magit adds using
+~add-hook~.  In doing so you have to make sure you do not attempt to
+remove function that haven't even been added yet, for example:
+
+#+BEGIN_SRC emacs-lisp
+  (with-eval-after-load 'magit-diff
+    (remove-hook 'magit-section-movement-hook
+                 'magit-hunk-set-window-start))
+#+END_SRC
+
 - Variable: magit-section-movement-hook
 
   This hook is run by all of the above movement commands, after
@@ -930,7 +940,9 @@ Any of the functions listed below can be used as members of this hook.
 
   This hook function ensures that the beginning of the current section
   is visible, provided it is a ~hunk~ section.  Otherwise, it does
-  nothing.  This function is a member of the hook's default value.
+  nothing.
+
+  Loading ~magit-diff~ adds this function to the hook.
 
 - Function: magit-section-set-window-start
 
@@ -943,15 +955,17 @@ Any of the functions listed below can be used as members of this hook.
 
   This hook function only has an effect in log buffers, and ~point~ is
   on the "show more" section.  If that is the case, then it doubles
-  the number of commits that are being shown.  This function is a
-  member of the hook's default value.
+  the number of commits that are being shown.
+
+  Loading ~magit-log~ adds this function to the hook.
 
 - Function: magit-log-maybe-update-revision-buffer
 
   When moving inside a log buffer, then this function updates the
   revision buffer, provided it is already being displayed in another
-  window of the same frame.  This function is a member of the hook's
-  default value.
+  window of the same frame.
+
+  Loading ~magit-log~ adds this function to the hook.
 
 - Function: magit-log-maybe-update-blob-buffer
 
@@ -1067,7 +1081,7 @@ useful.  But your mileage may vary.
 
 When a buffer is first created then some sections are shown expanded
 while others are not.  This is hard coded.  When a buffer is refreshed
-then the previous visibility is preserved. The initial visibility of
+then the previous visibility is preserved.  The initial visibility of
 certain sections can also be overwritten using the hook
 ~magit-section-set-visibility-hook~.
 

Documentation/magit.texi

@@ -31,7 +31,7 @@ General Public License for more details.
 @finalout
 @titlepage
 @title Magit User Manual
-@subtitle for version 2.90.1 (v2.90.1-880-gf63077242+1)
+@subtitle for version 2.90.1 (v2.90.1-895-g1f79b3d77+1)
 @author Jonas Bernoulli
 @page
 @vskip 0pt plus 1filll
@@ -53,7 +53,7 @@ directly from within Emacs.  While many fine Git clients exist, only
 Magit and Git itself deserve to be called porcelains.
 
 @noindent
-This manual is for Magit version 2.90.1 (v2.90.1-880-gf63077242+1).
+This manual is for Magit version 2.90.1 (v2.90.1-895-g1f79b3d77+1).
 
 @quotation
 Copyright (C) 2015-2020 Jonas Bernoulli <jonas@@bernoul.li>
@@ -1358,6 +1358,16 @@ Move to the beginning of the parent of the current section.
 The above commands all call the hook @code{magit-section-movement-hook}.
 Any of the functions listed below can be used as members of this hook.
 
+You might want to remove some of the functions that Magit adds using
+@code{add-hook}.  In doing so you have to make sure you do not attempt to
+remove function that haven't even been added yet, for example:
+
+@lisp
+(with-eval-after-load 'magit-diff
+  (remove-hook 'magit-section-movement-hook
+               'magit-hunk-set-window-start))
+@end lisp
+
 @defvar magit-section-movement-hook
 
 This hook is run by all of the above movement commands, after
@@ -1368,7 +1378,9 @@ arriving at the destination.
 
 This hook function ensures that the beginning of the current section
 is visible, provided it is a @code{hunk} section.  Otherwise, it does
-nothing.  This function is a member of the hook's default value.
+nothing.
+
+Loading @code{magit-diff} adds this function to the hook.
 @end defun
 
 @defun magit-section-set-window-start
@@ -1383,16 +1395,18 @@ variant in turn.
 
 This hook function only has an effect in log buffers, and @code{point} is
 on the "show more" section.  If that is the case, then it doubles
-the number of commits that are being shown.  This function is a
-member of the hook's default value.
+the number of commits that are being shown.
+
+Loading @code{magit-log} adds this function to the hook.
 @end defun
 
 @defun magit-log-maybe-update-revision-buffer
 
 When moving inside a log buffer, then this function updates the
 revision buffer, provided it is already being displayed in another
-window of the same frame.  This function is a member of the hook's
-default value.
+window of the same frame.
+
+Loading @code{magit-log} adds this function to the hook.
 @end defun
 
 @defun magit-log-maybe-update-blob-buffer
@@ -1554,7 +1568,7 @@ Toggle visibility of bodies of children of the current section.
 
 When a buffer is first created then some sections are shown expanded
 while others are not.  This is hard coded.  When a buffer is refreshed
-then the previous visibility is preserved. The initial visibility of
+then the previous visibility is preserved.  The initial visibility of
 certain sections can also be overwritten using the hook
 @code{magit-section-set-visibility-hook}.
 

lisp/magit-diff.el

@@ -2244,6 +2244,8 @@ section or a child thereof."
           magit-diff-expansion-threshold)
        'hide))
 
+(add-hook 'magit-section-set-visibility-hook #'magit-diff-expansion-threshold)
+
 ;;; Revision Mode
 
 (define-derived-mode magit-revision-mode magit-diff-mode "Magit Rev"
@@ -2768,6 +2770,9 @@ actually a `diff' but a `diffstat' section."
 
 ;;; Diff Highlight
 
+(add-hook 'magit-section-unhighlight-hook #'magit-diff-unhighlight)
+(add-hook 'magit-section-highlight-hook #'magit-diff-highlight)
+
 (defun magit-diff-unhighlight (section selection)
   "Remove the highlighting of the diff-related SECTION."
   (when (magit-hunk-section-p section)

lisp/magit-log.el

@@ -1289,6 +1289,8 @@ exists mostly for backward compatibility reasons."
     (forward-line -1)
     (magit-section-forward)))
 
+(add-hook 'magit-section-movement-hook #'magit-log-maybe-show-more-commits)
+
 (defvar magit--update-revision-buffer nil)
 
 (defun magit-log-maybe-update-revision-buffer (&optional _)
@@ -1297,6 +1299,8 @@ If there is no revision buffer in the same frame, then do nothing."
   (when (derived-mode-p 'magit-log-mode 'magit-cherry-mode 'magit-reflog-mode)
     (magit--maybe-update-revision-buffer)))
 
+(add-hook 'magit-section-movement-hook #'magit-log-maybe-update-revision-buffer)
+
 (defun magit--maybe-update-revision-buffer ()
   (when-let ((commit (magit-section-value-if 'commit))
              (buffer (magit-get-mode-buffer 'magit-revision-mode nil t)))

lisp/magit-section.el

@@ -43,7 +43,7 @@
 (defgroup magit-section nil
   "Expandable sections."
   :link '(info-link "(magit)Sections")
-  :group 'magit)
+  :group 'extensions)
 
 (defcustom magit-section-show-child-count t
   "Whether to append the number of children to section headings.
@@ -52,63 +52,31 @@ This only applies to sections for which doing so makes sense."
   :group 'magit-section
   :type 'boolean)
 
-(defcustom magit-section-movement-hook
-  '(magit-hunk-set-window-start
-    magit-log-maybe-update-revision-buffer
-    magit-log-maybe-show-more-commits)
+(defvar magit-section-movement-hook nil
   "Hook run by `magit-section-goto'.
-That function in turn is used by all section movement commands."
-  :package-version '(magit . "2.3.0")
-  :group 'magit-section
-  :type 'hook
-  :options '(magit-hunk-set-window-start
-             magit-status-maybe-update-revision-buffer
-             magit-status-maybe-update-stash-buffer
-             magit-status-maybe-update-blob-buffer
-             magit-log-maybe-update-revision-buffer
-             magit-log-maybe-update-blob-buffer
-             magit-log-maybe-show-more-commits
-             magit-stashes-maybe-update-stash-buffer))
-
-(defcustom magit-section-highlight-hook
-  '(magit-diff-highlight
-    magit-section-highlight
+That function in turn is used by all section movement commands.")
+
+(defvar magit-section-highlight-hook
+  '(magit-section-highlight
     magit-section-highlight-selection)
   "Functions used to highlight the current section.
 Each function is run with the current section as only argument
-until one of them returns non-nil."
-  :package-version '(magit . "2.1.0")
-  :group 'magit-section
-  :type 'hook
-  :options '(magit-diff-highlight
-             magit-section-highlight
-             magit-section-highlight-selection))
+until one of them returns non-nil.")
 
-(defcustom magit-section-unhighlight-hook
-  '(magit-diff-unhighlight)
+(defvar magit-section-unhighlight-hook nil
   "Functions used to unhighlight the previously current section.
 Each function is run with the current section as only argument
 until one of them returns non-nil.  Most sections are properly
 unhighlighted without requiring a specialized unhighlighter,
-diff-related sections being the only exception."
-  :package-version '(magit . "2.1.0")
-  :group 'magit-section
-  :type 'hook
-  :options '(magit-diff-unhighlight))
+diff-related sections being the only exception.")
 
-(defcustom magit-section-set-visibility-hook
-  '(magit-diff-expansion-threshold
-    magit-section-cached-visibility)
+(defvar magit-section-set-visibility-hook
+  '(magit-section-cached-visibility)
   "Hook used to set the initial visibility of a section.
 Stop at the first function that returns non-nil.  The returned
 value should be `show', `hide' or nil.  If no function returns
 non-nil, determine the visibility as usual, i.e. use the
-hardcoded section specific default (see `magit-insert-section')."
-  :package-version '(magit . "2.4.0")
-  :group 'magit-section
-  :type 'hook
-  :options '(magit-diff-expansion-threshold
-             magit-section-cached-visibility))
+hardcoded section specific default (see `magit-insert-section').")
 
 (defcustom magit-section-cache-visibility t
   "Whether to cache visibility of sections.
@@ -591,6 +559,8 @@ It the SECTION has a different type, then do nothing."
   (when (magit-hunk-section-p section)
     (magit-section-set-window-start section)))
 
+(add-hook 'magit-section-movement-hook #'magit-hunk-set-window-start)
+
 (defmacro magit-define-section-jumper (name heading type &optional value)
   "Define an interactive function to go some section.
 Together TYPE and VALUE identify the section.