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>

git-rebase.sh

@@ -150,6 +150,13 @@ run_specific_rebase () {
 		autosquash=
 	fi
 	. git-rebase--$type
+	ret=$?
+	if test $ret -eq 0
+	then
+		git gc --auto &&
+		rm -rf "$state_dir"
+	fi
+	exit $ret
 }
 
 run_pre_rebase_hook () {