add tests for merge message headings

When calling "git merge $X", we automatically generate a
commit message containing something like "Merge branch
'$X'". This test script checks that those messages say what
they should, and exposes a failure when merging a refname
that is ambiguous between a tag and a branch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7608-merge-messages.sh

@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='test auto-generated merge messages'
+. ./test-lib.sh
+
+check_oneline() {
+	echo "$1" | sed "s/Q/'/g" >expect &&
+	git log -1 --pretty=tformat:%s >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'merge local branch' '
+	test_commit master-1 &&
+	git checkout -b local-branch &&
+	test_commit branch-1 &&
+	git checkout master &&
+	test_commit master-2 &&
+	git merge local-branch &&
+	check_oneline "Merge branch Qlocal-branchQ"
+'
+
+test_expect_success 'merge octopus branches' '
+	git checkout -b octopus-a master &&
+	test_commit octopus-1 &&
+	git checkout -b octopus-b master &&
+	test_commit octopus-2 &&
+	git checkout master &&
+	git merge octopus-a octopus-b &&
+	check_oneline "Merge branches Qoctopus-aQ and Qoctopus-bQ"
+'
+
+test_expect_success 'merge tag' '
+	git checkout -b tag-branch master &&
+	test_commit tag-1 &&
+	git checkout master &&
+	test_commit master-3 &&
+	git merge tag-1 &&
+	check_oneline "Merge commit Qtag-1Q"
+'
+
+test_expect_failure 'ambiguous tag' '
+	git checkout -b ambiguous master &&
+	test_commit ambiguous &&
+	git checkout master &&
+	test_commit master-4 &&
+	git merge ambiguous &&
+	check_oneline "Merge commit QambiguousQ"
+'
+
+test_done