is ORIG_HEAD allowed to point to a non-existing object?

From: Christian Halstrick <christian.halstrick@gmail.com>
Date: May 10, 2016, 12:15 UTC
Message-ID: <CAENte7jNxM7J9bWUnrOkLQJZ=8JEtYzw9-uxVkq-CpvGyk9UUw@mail.gmail.com>

If I do a "git-rebase -i ..." followed by "git reflog expire ..." and "git gc ..." then I can end up with a repo which has a ref ORIG_HEAD which points to a non-existing object.

- Is this intended?
- What's the reason to keep a ref which points to a non-existing object?
- Are there any other refs which are allowed to point to non-existing objects?

Here is how I reproduced the problem:

> git init

Initialized empty Git repository in /tmp/yy/.git/

> touch a
> git add a
> git commit -m addA

[master (root-commit) ff82f68] addA
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a

> echo 2 >a
> git commit -a -m modA

[master f9ae5a7] modA
 1 file changed, 1 insertion(+)

> echo 3 >a
> git commit -a -m 'fixup! modA'

[master 271012b] fixup! modA
 1 file changed, 1 insertion(+), 1 deletion(-)

> git rebase -i --autosquash HEAD~~

[detached HEAD 3eb17d4] modA
 1 file changed, 1 insertion(+)
Successfully rebased and updated refs/heads/master.

> git rev-parse --short ORIG_HEAD

271012b

> git reflog expire --expire=now --all
> git rev-parse 271012b

271012be467fcbd62a875b7187c0e9871f62ac0d

> git gc --prune=now

Counting objects: 6, done. Delta compression using up to 2 threads. Compressing objects: 100% (2/2), done. Writing objects: 100% (6/6), done. Total 6 (delta 1), reused 0 (delta 0)

> git rev-parse 271012b

271012b fatal: ambiguous argument '271012b': unknown revision or path not in the working tree. Use '--' to separate paths from revisions, like this: 'git <command> [<revision>...] -- [<file>...]'

> git rev-parse --short ORIG_HEAD

271012b

Ciao
  Chris