rebase: prepare to do generic housekeeping

On successful completion of a rebase in git-rebase--$backend.sh, the $backend script cleans up on its own and exits. The cleanup routine is however, independent of the $backend, and each $backend script unnecessarily duplicates this work: rm -rf "$state_dir" git gc --auto Prepare git-rebase.sh for later patches that return control from each $backend script back to us, for performing this generic cleanup routine. The code that this patch adds is currently unreachable, and will only start to be used when git-rebase--$backend.sh scripts are taught to return control in later patches. Another advantage is that git-rebase.sh can implement a generic finish_rebase() to possibly do additional tasks in addition to the cleanup. Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>