PYCBC-220: Check explicitly that we're a git repo

Change-Id: I7fa06324b545a9b6ff7073b769b46e08134423be
Reviewed-on: http://review.couchbase.org/32386
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>