git-p4: respect excluded paths when detecting branches

git-p4: respect excluded paths when detecting branches

Currently, excluded paths are only handled in the following cases:
 * no branch detection;
 * branch detection with using clientspec.

However, excluded paths are not respected in case of
branch detection without using clientspec.

Fix this by consulting the list of excluded paths
when splitting files across branches.

Signed-off-by: Andrey Mazo <amazo@checkvideo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>