Don't try to load git_pillar if not enabled in master config

This fixes #6052 by doing a check for git-based external pillars in the
master config, and returning False when there are none. This suppresses
an error later on in the __virtual__() function which complains that the
git fileserver backend is configured (when it clearly is not).
Correct innacuracy in log message

This log entry was apparently copied directly from the git fileserver
backend, but this isn't the git fileserver backend, it's the git
external pillar.
Change gitpython to GitPython

Use the proper capitalization of GitPython.