Allow gitpython gitfs_provider to use git shas as envs

This only works so far for the fileclient, it will not work in the top
file. Additional work needs to be done to enable sha envs to be found by
the top file.
Allow pygit2 gitfs_provider to use git shas as envs

This extends this feature (added for gitpython at last night's sprint)
to pygit2.