t3308: create a real ref directory/file conflict

A test in t3308 wants to make sure that we don't accidentally merge into "refs/notes/dir" when it exists as a directory, so it does: mkdir .git/refs/notes/dir git -c core.notesRef=refs/notes/dir merge ... and expects the second command to fail. But that understimates the refs code, which is smart enough to remove useless directories in the refs hierarchy. The test succeeded only because of a bug which prevented resolving refs/notes/dir for writing, even though an actual ref update would succeed. In preparation for fixing that bug, let's switch to creating a real ref in refs/notes/dir, which is a more realistic situation. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>