share/zoneinfo: update import documentation for Git

Document the steps needed to import and MFC new versions of tzdata now
that FreeBSD has moved from Subversion to Git.

share/zoneinfo/Makefile

@@ -3,29 +3,31 @@
 #
 # HOW TO UPDATE THE ZONEINFO DATA
 #
-# With the use of subversion, this is a little bit simpler than the CVS method.
-#
 # Import the new sources to the vendor branch:
 #
-# $ cd ~/svn/vendor/tzdata/dist
-# $ tar zxvf /usr/ports/distfile/tzdata2008X.tar.gz
-#   (check with "svn stat" and "svn diff" if it all makes sense)
-# $ svn commit	# Commit message: "Vendor import of tzdata2008X (+details)" 
+# $ cd ~/freebsd/src
+# $ git worktree add ../tzdata vendor/tzdata
+# $ pushd ../tzdata
+# $ tar -xvf ../tzdata-latest.tar.gz
+#   (check with "git status" and "git diff" if it all makes sense)
+# $ git add -A
+# $ git commit -m "Import tzdata 20XXX"
+# $ git tag -a -m "Tag import of tzdata 20XXX"
+# $ git push --follow-tags freebsd vendor/tzdata
+# $ popd
 #
-# Tag it
+# Merge-from-vendor
 #
-# $ cd ~/svn/vendor/tzdata
-# $ svn cp svn+ssh://svn.freebsd.org/base/vendor/tzdata/dist \
-#	svn+ssh://svn.freebsd.org/base/vendor/tzdata/tzdata2008X
-# $ svn commit	# Commit message: "Tag of tzdata2008X"
+# $ git subtree merge -P contrib/tzdata vendor/tzdata
+#   (write a meaningful commit message)
+# $ git push freebsd HEAD:main
 #
-# Merge-from-vendor
+# MFC
 #
-# $ cd ~/svn/head/contrib/tzdata
-# $ svn update
-# $ svn merge -c X --accept=postpone \
-#	svn+ssh://svn.freebsd.org/base/vendor/tzdata/dist .
-# $ svn commit	# Commit message: "MFV of tzdata2008X"
+# $ git checkout -b freebsd/stable/12 stable-12
+# $ git cherry-pick -x [hash of merge commit to main] -m 1 --edit
+#   (write a meaningful commit message)
+# $ git push freebsd HEAD:stable/12
 #
 
 .include <src.opts.mk>