git-p4: Allow filtering Perforce branches by user

git-p4: Allow filtering Perforce branches by user

All branches in the Perforce server are downloaded to allow branch detection. If
you have a centralized server on a remote location and there is a big number of
branches this operation can take some time.
This patch adds the configuration option git-p4.branchUser to allow filtering
the branch list by user. Although this limits the branch maintenance in Perforce
to be done by a single user, it might be an advantage when the number of
branches being used in a specific depot is very small when compared with the
branches available in the server.

Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>