merge: do not mistake (ancestor of) tag for branch

If no branch 'foo' exists but a tag 'foo' does, then
git merge foo^ results in

	Merge branch 'foo' (early part)

as a commit message, because the relevant code path checks that
refs/heads/foo is a valid refname for writing rather than for
reading.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -437,7 +437,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 		strbuf_addstr(&truname, "refs/heads/");
 		strbuf_addstr(&truname, remote);
 		strbuf_setlen(&truname, truname.len - len);
-		if (resolve_ref(truname.buf, buf_sha, 0, NULL)) {
+		if (resolve_ref(truname.buf, buf_sha, 1, NULL)) {
 			strbuf_addf(msg,
 				    "%s\t\tbranch '%s'%s of .\n",
 				    sha1_to_hex(remote_head->sha1),

t/t7600-merge.sh

@@ -558,8 +558,11 @@ test_expect_success 'refresh the index before merging' '
 	git merge c3
 '
 
-cat >expected <<EOF
-Merge branch 'c5' (early part)
+cat >expected.branch <<\EOF
+Merge branch 'c5-branch' (early part)
+EOF
+cat >expected.tag <<\EOF
+Merge commit 'c5~1'
 EOF
 
 test_expect_success 'merge early part of c2' '
@@ -577,9 +580,14 @@ test_expect_success 'merge early part of c2' '
 	git add c6.c &&
 	git commit -m c6 &&
 	git tag c6 &&
+	git branch -f c5-branch c5 &&
+	git merge c5-branch~1 &&
+	git show -s --pretty=format:%s HEAD >actual.branch &&
+	git reset --keep HEAD^ &&
 	git merge c5~1 &&
-	git show -s --pretty=format:%s HEAD > actual &&
-	test_cmp actual expected
+	git show -s --pretty=format:%s HEAD >actual.tag &&
+	test_cmp expected.branch actual.branch &&
+	test_cmp expected.tag actual.tag
 '
 
 test_debug 'gitk --all'