git-p4: Fix direct import from perforce after fetching changes through git from origin

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>