When using an existing git repository to cache the perforce import we don't
fetch the branch mapping from perforce as that is a slow operation. However
the origin repository may not be fully up-to-date and therefore it may be
necessary to import more changes directly from Perforce.
Such a direct import needs self.knownBranches to be set up though, so
initialize it from the existing p4/* git branches.
Signed-off-by: Simon Hausmann <simon@lst.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>