rebase: prepare to do generic housekeeping

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>