rebase: finish_rebase() in fast-forward rebase

In the following case

  $ git rebase master
  Fast-forwarded autostash-fix to master.

The autostash is not applied automatically, because this codepath
forgets to call finish_rebase().  Fix this.  Also add a test to guard
against regressions.

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

git-rebase.sh

@@ -579,6 +579,7 @@ if test "$mb" = "$orig_head"
 then
 	say "$(eval_gettext "Fast-forwarded \$branch_name to \$onto_name.")"
 	move_to_original_branch
+	finish_rebase
 	exit 0
 fi