Fix git-p4 for newer versions of Perforce.

Different versions of perforce / git appear to use various naming
schemes for perforce variables. This fix updates the regular expression
to match two known names for the perforce path variable.

Fixes bug #3169

Reviewed at https://reviews.reviewboard.org/r/5120/
Fix entry point lookups without any custom entry points.

The recent fix for looking up third party entry points was failing if
there weren't any custom entry points registered. Typing a command that
RBTools doesn't supply would raise a StopIteration error. This happened
because the lookup assumed the existence of at least one entry.

It's now a bit more bullet-proof and catches that StopIteration
cleanly.