Fix perforce posting from subdirectories of the client root.

The logic that checked whether we were inside a p4 client was adding
os.path.pathsep to the path name instead of os.path.sep. This meant that we'd
only match if the cwd was the client root.

Testing done:
* Made a subdirectory in a p4 client and posted a change.

Fixes bug 3218.

Reviewed at https://reviews.reviewboard.org/r/5390/