Updated openstack/openstack
Project: openstack-dev/pbr  50a43a18d14f4c2acebbd3fb221400224eb56781

Properly check for git before getting git dir

We cannot get the git dir if git is not installed. Check that git is
installed before querying for the git dir. Return None if the git dir
cannot be found or if git is not installed.

Change-Id: Ic8e0c74a779b23842369a8cf01fcbf37885202ef
Fixes-bug: 1326682
(cherry picked from commit e7d2825d39fce2111d0d413e63e553603a0e56fb)