git p4: make branch detection work with --use-client-spec

git p4: make branch detection work with --use-client-spec

The bug report in http://stackoverflow.com/questions/11893688
observes that files are mapped into the wrong locations in
git when both --use-client-spec and --branch-detection are enabled.

Fix this by changing the relative path prefix to match discovered
branches when using a client spec.

The problem was likely introduced with ecb7cf9 (git-p4: rewrite view
handling, 2012-01-02).

Signed-off-by: Pete Wyckoff <pw@padd.com>
Tested-by: Matthew Korich <matthew@korich.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>