git-rebase.txt: add a note about 'ORIG_HEAD' being overwritten

'ORIG_HEAD' is written at the start of the rebase, but is not guaranteed
to still point to the original branch tip at the end of the rebase.

Indeed, using other commands that write 'ORIG_HEAD' during the rebase,
like splitting a commit using 'git reset HEAD^', will lead to 'ORIG_HEAD'
being overwritten. This causes confusion for some users [1].

Add a note about that in the 'Description' section, and mention the more
robust alternative of using the branch's reflog.

[1] https://lore.kernel.org/git/[email protected]/T/#m827179c5adcfb504d67f76d03c8e6942b55e5ed0

Reported-by: Erik Cervin Edin <[email protected]>
Signed-off-by: Philippe Blain <[email protected]>
Acked-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

Documentation/git-rebase.txt

@@ -38,6 +38,13 @@ The current branch is reset to `<upstream>` or `<newbase>` if the
 `git reset --hard <upstream>` (or `<newbase>`). `ORIG_HEAD` is set
 to point at the tip of the branch before the reset.
 
+[NOTE]
+`ORIG_HEAD` is not guaranteed to still point to the previous branch tip
+at the end of the rebase if other commands that write that pseudo-ref
+(e.g. `git reset`) are used during the rebase. The previous branch tip,
+however, is accessible using the reflog of the current branch
+(i.e. `@{1}`, see linkgit:gitrevisions[7]).
+
 The commits that were previously saved into the temporary area are
 then reapplied to the current branch, one by one, in order. Note that
 any commits in `HEAD` which introduce the same textual changes as a commit