[git/contrib/subtree] Remote-ref unexpectedly deleted if pushing subtree commit twice

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

# setup main and subtree repo $ git init main && git init subtree $ cd main && git remote add subtr ../subtree/.git

# populate main repo $ mkdir dir && echo hello > dir/hello.txt $ git add . && git commit -m "Commit dir"

# split/push subtree 'dir'... $ git subtree push --prefix=dir --rejoin --squash subtr subtrbranch # ...and again $ git subtree push --prefix=dir --rejoin --squash subtr subtrbranch

What did you expect to happen? (Expected behavior)

- No change on 2nd  git subtree push

What happened instead? (Actual behavior)

- Remote-ref unexpectedly deleted on 2nd git subtree push:
 > git push using:  subtr subtrbranch
 > Subtree is already at commit xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.
 > To ../subtree/.git
 >  - [deleted]         subtrbranch

What's different between what you expected and what actually happened?

- No change vs deleted remote-ref on 2nd  git subtree push.

Anything else you want to add:

- Root cause in contrib/subtree/git-subtree.sh (branch next, 3824d2c):
   In the scenario above:
   cmd_push calls cmd_split in a subshell:
         localrev=$(cmd_split "$localrev_presplit" "$repository") || die
   cmd_split calls cmd_merge.
   cmd_merge exits 0 without writing to stdout
         if test "$sub" = "$rev"
         then
             say >&2 "Subtree is already at commit $rev."
             exit 0
         fi
   With exit 0 only the subshell is exited. Instead of dying this 
assignement
         localrev=$(cmd_split "$localrev_presplit" "$repository") || die
   becomes
         localrev=
   yielding a refspec  :refs/heads/$remoteref  pushed to the remote 
repository:
         git push "$repository" "$localrev":"refs/heads/$remoteref"

- Proposed fix (I can try to submit a patch if desired):