t4150: am fails if index is dirty

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
will ensure that the index is clean before applying the patch. This is
to prevent changes unrelated to the patch from being committed.

Add a test for this check.

Reviewed-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4150-am.sh

@@ -154,6 +154,18 @@ test_expect_success 'am applies patch correctly' '
 	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
 '
 
+test_expect_success 'am fails if index is dirty' '
+	test_when_finished "rm -f dirtyfile" &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout first &&
+	echo dirtyfile >dirtyfile &&
+	git add dirtyfile &&
+	test_must_fail git am patch1 &&
+	test_path_is_dir .git/rebase-apply &&
+	test_cmp_rev first HEAD
+'
+
 test_expect_success 'am applies patch e-mail not in a mbox' '
 	rm -fr .git/rebase-apply &&
 	git reset --hard &&