git p4: allow short ref names to --branch

git p4: allow short ref names to --branch

For a clone or sync, --branch says where the newly imported
branch should go, or which existing branch to sync up.  It
takes an argument, which is currently either something that
starts with "refs/", or if not, "refs/heads/p4" is prepended.

Putting it in heads seems like a bad default; these should
go in remotes/p4/ in most situations.  Make that the new default,
and be more liberal in the form of the branch name.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>