validate_headref: tighten ref-matching to just branches

validate_headref: tighten ref-matching to just branches

When we are trying to determine whether a directory contains
a git repository, one of the tests we do is to check whether
HEAD is either a symlink or a symref into the "refs/"
hierarchy, or a detached HEAD.

We can tighten this a little more, though: a non-detached
HEAD should always point to a branch (since checking out
anything else should result in detachment), so it is safe to
check for "refs/heads/".

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