am: return control to caller, for housekeeping

We only need to do these two tasks

git gc --auto
rm -fr "$dotest"

ourselves if the script was invoked as a standalone program; when
invoked with --rebasing (from git-rebase--am.sh), cascade control back
to the ultimate caller git-rebase.sh to do this for us.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>