t5505: fix "set-head --auto with ambiguous HEAD" test

When two or more branches point at the same commit and HEAD is
pointing at one of them, without the symref extension, there is no
way to remotely tell which one of these branches HEAD points at.
The test in question attempts to make sure that this situation is
diagnosed and results in a failure.

However, even if there _were_ a way to reliably tell which branch
the HEAD points at, "set-head --auto" would fail if there is no
remote tracking branch.  Make sure that this test does not fail
for that "wrong" reason.

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

t/t5505-remote.sh

@@ -271,6 +271,7 @@ EOF
 test_expect_success 'set-head --auto fails w/multiple HEADs' '
 	(
 		cd test &&
+		git fetch two "refs/heads/*:refs/remotes/two/*" &&
 		test_must_fail git remote set-head --auto two >output 2>&1 &&
 		test_i18ncmp expect output
 	)