git p4: clone --branch should checkout master

When using the --branch argument to "git p4 clone", one
might specify a destination for p4 changes different from
the default refs/remotes/p4/master.  Both cases should
create a master branch and checkout files.

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