subtree: t7900: add a test for the -h flag

It's a dumb test, but it's surprisingly easy to break.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/t/t7900-subtree.sh

@@ -37,6 +37,13 @@ last_commit_subject () {
 	git log --pretty=format:%s -1
 }
 
+test_expect_success 'shows short help text for -h' '
+	test_expect_code 129 git subtree -h >out 2>err &&
+	test_must_be_empty err &&
+	grep -e "^ *or: git subtree pull" out &&
+	grep -e --annotate out
+'
+
 #
 # Tests for 'git subtree add'
 #