FAQ: detailed example for the "accidentally changed files in the wron…

…g commit" question

It was requested on Discord once.

The result is almost a tutorial; we could consider actually moving it into the
tutorial and providing a link in the FAQ. That could be done separately.

The `jj` commands outputs are faked in a few, hopefully inconspicuous, ways.
Hopefully, this is not distracting.

docs/FAQ.md

@@ -298,14 +298,184 @@ jj config set --user git.private-commits "'''description(glob:'private:*')'''"
 
 ### I accidentally changed files in the wrong commit, how do I move the recent changes into another commit?
 
-Use `jj evolog -p` to see how your working-copy commit has evolved. Find the
-commit you want to restore the contents to. Let's say the current commit (with
-the changes intended for a new commit) are in commit X and the state you wanted
-is in commit Y. Note the commit id (normally in blue at the end of the line in
-the log output) of each of them. Now use `jj new` to create a new working-copy
-commit, then run `jj restore --from Y --into @-` to restore the parent commit
-to the old state, and `jj restore --from X` to restore the new working-copy
-commit to the new state.
+Let's say we are editing a commit for "featureA", and we forgot to run `jj
+new` or `jj commit` before doing some work that belongs in a new commit:
+
+```console
+$ jj log
+@  lnvvtrzo [email protected] 2025-02-28 21:01:10 31a347e0
+│  featureA
+◆  zzzzzzzz root() 00000000
+$ cat file  # Oh no, the work on "feature B" should be in a separate commit!
+Done with feature A
+Working on feature B
+```
+
+#### Step 1: Find the commit id for the "last good version"
+
+<!-- TODO: Reorganize the two related questions, this one and
+  -- the one linked below
+  -->
+You can find [all the past versions of the working copy revision that `jj` has
+saved](#jj-is-said-to-record-the-working-copy-after-jj-log-and-every-other-command-where-can-i-see-these-automatic-saves)
+by running `jj evolog`. The obsolete versions will be marked as "hidden" and
+will have the same change id, but will have different commit ids. This
+represents different [commits] that are all parts of the same [change].
+
+For example, this is what the evolog might look like after you made two edits to
+the same change:
+
+```console
+$ # Note the word "hidden", the commit ids on the right,
+$ # and the unchanging change id on the left.
+$ jj evolog
+@  lnvvtrzo [email protected] 2025-02-28 21:01:10 31a347e0
+│  featureA
+○  lnvvtrzo hidden [email protected] 2025-02-28 21:00:51 b8004ab8
+│  featureA
+○  lnvvtrzo hidden [email protected] 2025-02-28 20:50:05 e4d831d
+   (no description set)
+```
+
+Since commit `b800` is hidden, it is considered obsolete and `jj log` (without
+arguments) will not show it, nor can it be accessed by its change id. However,
+most `jj` operations work normally on such commits if you refer to them by their
+commit id.
+
+To find out which of these versions is the last time before we started working
+on feature B (the point where we should have created a new change, but failed
+to do so), we can look at the actual changes between the `evolog` commits by
+running `jj evolog -p`:
+
+```console
+$ # When was the last saved point before we started working on feature B?
+$ jj evolog -p --git  # We use `--git` to make diffs clear without colors
+@  lnvvtrzo [email protected] 2025-02-28 21:01:10 31a347e0
+│  featureA
+│  diff --git a/file b/file
+│  index 2b455c4207..2a7e05a01a 100644
+│  --- a/file
+│  +++ b/file
+│  @@ -1,1 +1,2 @@
+│   Done with feature A
+│  +Working on feature B
+○  lnvvtrzo hidden [email protected] 2025-02-28 21:00:51 b8004ab8
+│  featureA
+│  diff --git a/file b/file
+│  index cb61245109..2b455c4207
+│  --- a/file
+│  +++ b/file
+│  @@ -1,1 +1,1 @@
+│  -Working on feature A
+│  +Done with feature A
+○  lnvvtrzo hidden [email protected] 2025-02-28 20:50:05 e4d831d
+   (no description set)
+   diff --git a/file b/file
+   index 0000000000..cb61245109
+   --- /dev/null
+   +++ b/file
+   @@ 0,0 +1,1 @@
+   +Working on feature A
+```
+
+The version of the change when we were actually done with "featureA" thus
+corresponds to the commit with id `b800`. The following edit (commit `31a3`)
+belongs in a new change.
+
+#### Step 2: Create a new change for the current state and restore the existing change to the older state
+
+The "featureA" cahnge is currently at commit `31a`:
+
+```console
+$ jj log
+@  lnvvtrzo [email protected] 2025-02-28 21:01:10 31a347e0
+│  featureA
+◆  zzzzzzzz root() 00000000
+```
+
+We'd like to create a new "featureB" change with the contents of the current
+commit `31a`, and we'd the "featureA" change to be reverted to its former
+state at commit `b80` (see step 1 above for how we found that commit id).
+
+First, we create a new empty child commit. Since it is empty, it has the same
+contents as `31a`.
+
+```console
+$ jj new -m "featureB"
+Working copy now at: pvnrkl 47171aa (empty) featureB
+Parent commit      : lnvvtr 31a347e featureA
+$ cat file
+Done with feature A
+Working on feature B
+```
+
+Now, we `jj restore` the change `lnvvtr` to its state at commit `b80`. We use
+the `--restore-descendants` flag so that the *file contents* (AKA snapshot) of
+the "featureB" change is preserved.
+
+```console
+$ # We refer to `lnvvtr` as `@-` for brevity
+$ jj restore --from b80 --into @- --restore-descendants
+Created lnvvtr 599994e featureA
+Rebased 1 descendant commits (while preserving their content)
+Working copy now at: pvnrkl 468104c featureB
+Parent commit      : lnvvtr 599994e featureA
+```
+
+Even though `@-` was modified, `--restore-descendants` preserved the contents of
+the current change:
+
+```console
+$ jj file show -r @ file  # Same as `cat file`
+Done with feature A
+Working on feature B
+$ jj file show -r @- file
+Done with feature A
+```
+
+??? info "More details on what `--restore-descendants` does"
+
+    When we ran the `jj restore` command, the working copy change `@` was
+    at commit `471` and `@` was the only child of `@-`. In this situation,
+
+    ```shell
+    jj restore --from b80 --into @- --restore-descendants
+    ```
+
+    is equivalent to
+
+    ```shell
+    jj restore --from b80 --into @-
+    jj restore --from 471 --into @
+    ```
+
+Now, we have achieved the exact state we desired:
+
+```
+$ jj log -p --git
+@  pvnrklkn [email protected] 2025-02-28 21:39:29 468104c2
+│  featureB
+│  diff --git a/file b/file
+│  index 2b455c4207..2a7e05a01a 100644
+│  --- a/file
+│  +++ b/file
+│  @@ -1,1 +1,2 @@
+│   Done with feature A
+│  +Working on feature B
+○  lnvvtrzo [email protected] 2025-02-28 21:39:29 599994ee
+│  featureA
+│  diff --git a/file b/file
+│  new file mode 100644
+│  index 0000000000..2b455c4207
+│  --- /dev/null
+│  +++ b/file
+│  @@ -0,0 +1,1 @@
+│  +Done with feature A
+◆  zzzzzzzz root() 00000000
+$ jj diff --from b80 --to @- # No output means these are identical
+$ jj diff --from 31a --to @  # No output means these are identical
+```
+
 
 ### How do I resume working on an existing change?
 
@@ -392,9 +562,11 @@ detect custom backends and more).
 
 [bookmarks_conflicts]: bookmarks.md#conflicts
 
+[change]: glossary.md#change
 [change ID]: glossary.md#change-id
 [co-located]: glossary.md#co-located-repos
 [commit ID]: glossary.md#commit-id
+[commits]: glossary.md#commit
 [config]: config.md
 
 [gerrit-integration]: https://gist.github.com/thoughtpolice/8f2fd36ae17cd11b8e7bd93a70e31ad6