t3308: create a real ref directory/file conflict

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>