rebase -i: stop overwriting ORIG_HEAD buffer

rebase -i: stop overwriting ORIG_HEAD buffer

After rebasing, ORIG_HEAD is supposed to point to the old HEAD of the
rebased branch.  The code used find_unique_abbrev() to obtain the
object name of the old HEAD and wrote to both
.git/rebase-merge/orig-head (used by `rebase --abort` to go back to
the previous state) and to ORIG_HEAD.  The buffer find_unique_abbrev()
gives back is volatile, unfortunately, and was overwritten after the
former file is written but before ORIG_FILE is written, leaving an
incorrect object name in it.

Avoid relying on the volatile buffer of find_unique_abbrev(), and
instead supply our own buffer to keep the object name.

I think that all of the users of head_hash should actually be using
opts->orig_head instead as passing a string rather than a struct
object_id around is a hang over from the scripted implementation. This
patch just fixes the immediate bug and adds a regression test based on
Caspar's reproduction example[1]. The users will be converted to use
struct object_id and head_hash removed in the next few commits.

[1] https://lore.kernel.org/git/CAFzd1+7PDg2PZgKw7U0kdepdYuoML9wSN4kofmB_-8NHrbbrHg@mail.gmail.com

Reported-by: Caspar Duregger <herr.kaste@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>