update to 2.28.0-rc0
    
    Release notes:
    https://github.com/git/git/raw/v2.28.0-rc0/Documentation/RelNotes/2.28.0.txt
    
    Update git.skip-test-patterns to catch the 2GB clone test.  The output
    of the skipped test was changed (for the better) in upstream commit
    d63ae31962 (t5608: avoid say() and use "skip_all" instead for
    consistency, 2020-05-22).