rebase -i: work on a detached HEAD

Earlier, rebase -i refused to rebase a detached HEAD.  Now it no longer
does.

Incidentally, this fixes "git gc --auto" shadowing the true exit status.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -309,4 +309,12 @@ test_expect_success '--continue tries to commit, even for "edit"' '
 	test $parent = $(git rev-parse HEAD^)
 '
 
+test_expect_success 'rebase a detached HEAD' '
+	grandparent=$(git rev-parse HEAD~2) &&
+	git checkout $(git rev-parse HEAD) &&
+	test_tick &&
+	FAKE_LINES="2 1" git rebase -i HEAD~2 &&
+	test $grandparent = $(git rev-parse HEAD~2)
+'
+
 test_done