git-p4: Make 'git-p4 branches' work after an initial clone with git clone from an origin-updated repository.

After a clone with "git clone" of a repository the p4 branches are only in remotes/origin/p4/* and not in remotes/p4/*.
Separate the code for detection and creation out of the P4Sync command class into standalone methods and use them
from the P4Branches command.
Signed-off-by: NSimon Hausmann <simon@lst.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>