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: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NJunio 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
 

t/t3420-rebase-autostash.sh

@@ -141,6 +141,17 @@ testrebase() {
 	'
 }
 
+test_expect_success "rebase: fast-forward rebase" '
+	test_config rebase.autostash true &&
+	git reset --hard &&
+	git checkout -b behind-feature-branch feature-branch~1 &&
+	test_when_finished git branch -D behind-feature-branch &&
+	echo dirty >>file1 &&
+	git rebase feature-branch &&
+	grep dirty file1 &&
+	git checkout feature-branch
+'
+
 testrebase "" .git/rebase-apply
 testrebase " --merge" .git/rebase-merge
 testrebase " --interactive" .git/rebase-merge