Make git-p4 support handle P4PORT environment variable.

The git-p4 support in rbtools only worked if you had git-p4.port in the git
config. If users had P4PORT in their environment instead of using this config
key, it wouldn't detect the repo as being a git-p4 clone.

Testing done: Used rbt post with my git-p4 clone.

Reviewed at https://reviews.reviewboard.org/r/5243/