rebase -i: handle "Nothing to do" case with autostash

rebase -i: handle "Nothing to do" case with autostash

When a user invokes

  $ git rebase -i @~3

with dirty files and rebase.autostash turned on, and exits the $EDITOR
with an empty buffer, the autostash fails to apply. Although the primary
focus of rr/rebase-autostash was to get the git-rebase--backend.sh
scripts to return control to git-rebase.sh, it missed this case in
git-rebase--interactive.sh. Since this case is unlike the other cases
which return control for housekeeping, assign it a special return status
and handle that return value explicitly in git-rebase.sh.

Reported-by: Karen Etheridge <ether@cpan.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>