Add pygit2 support to git fileserver backend
Add gitfs_provider to master config file template

Also moved a paragraph that seemed to be out of place
Add gitfs configuration parameters to master config docs

This commit adds the new gitfs_provider parameter, as well as several
other params for gitfs that were not documented.
Make gitfs prefer pygit2 if gitfs_provider not specified
Merge pull request #9566 from terminalmage/gitfs-pygit2

Add pygit2 support to git fileserver backend