t4150: am with applypatch-msg hook

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
will invoke the applypatch-msg hooks just after extracting the patch
message. If the applypatch-msg hook exits with a non-zero status, git-am
abort before even applying the patch to the index.

Add tests for this hook.

Helped-by: Junio C Hamano <gitster@pobox.com>
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

@@ -199,6 +199,41 @@ test_expect_success 'am applies patch e-mail with preceding whitespace' '
 	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
 '
 
+test_expect_success 'am with applypatch-msg hook' '
+	test_when_finished "rm -f .git/hooks/applypatch-msg" &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout first &&
+	mkdir -p .git/hooks &&
+	write_script .git/hooks/applypatch-msg <<-\EOF &&
+	cat "$1" >actual-msg &&
+	echo hook-message >"$1"
+	EOF
+	git am patch1 &&
+	test_path_is_missing .git/rebase-apply &&
+	git diff --exit-code second &&
+	echo hook-message >expected &&
+	git log -1 --format=format:%B >actual &&
+	test_cmp expected actual &&
+	git log -1 --format=format:%B second >expected &&
+	test_cmp expected actual-msg
+'
+
+test_expect_success 'am with failing applypatch-msg hook' '
+	test_when_finished "rm -f .git/hooks/applypatch-msg" &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout first &&
+	mkdir -p .git/hooks &&
+	write_script .git/hooks/applypatch-msg <<-\EOF &&
+	exit 1
+	EOF
+	test_must_fail git am patch1 &&
+	test_path_is_dir .git/rebase-apply &&
+	git diff --exit-code first &&
+	test_cmp_rev first HEAD
+'
+
 test_expect_success 'setup: new author and committer' '
 	GIT_AUTHOR_NAME="Another Thor" &&
 	GIT_AUTHOR_EMAIL="a.thor@example.com" &&