Improvements to the dist-git directive.
    
    Summary:
    Our use case is that we want to be able to run the `modlint` tool in
    response to dist-git commits to modulemd.yaml files.  `modlint` is kind
    of like `rpmlint`, except for module definitions.
    
    We want to run those checks in response to dist-git commits via
    taskotron-trigger.
    
    We were excited to see that a dist-git directive already exists, but it
    assumed that you were specifying your request in terms of an rpm NEVR
    which we do not have in our use case.
    
    This changes opens it up to be able to pull down any file from any
    namespace of dist-git repo, on any branch.  It is up to the caller of
    the dist-git directive to specify all of that information.
    
    Also, allow dist_git_commit as a possible item_type. This is for
    https://github.com/mmathesius/check_modulemd/pull/1
    
    Also, I originally filed this as https://phab.qadevel.cloud.fedoraproject.org/D1055,
    but I had issues updating the reviewers, so I've recreated it here.
    
    Test Plan:
        Run the test suite.
    
    Reviewers: tflink, mkrizek, kparal, jskladan
    
    Reviewed By: jskladan
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1069