Add a trigger for dist-git commit events.
    
    Summary:
    This lets us add tests that trigger in response to dist-git commits.
    
    I originally wrote this with a single `message_type` for all namespaces,
    but then found that I didn't have a way in the trigger rules to specify
    that a test (like `modlint`) should be run on all module dist-git
    commits but not on all rpm dist-git commits.  To fix this, I created
    three separate message types, one for each of the namespaces we
    currently have.
    
    This of course means that we can write taskotron checks that check our
    taskotron checks.
    
    Test Plan: Run the test suite.
    
    Reviewers: tflink, mkrizek, jskladan
    
    Reviewed By: jskladan
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1064