Drop git-gnome-keyring obsolete for fedora > 30
    
    Add a conditional to allow merging from master to f29.  The obsoletes
    should be removed when f29 is EOL.  It was added in 2d1c8b1 ("Remove
    obsolete gnome-keyring credential helper", 2018-01-09).  The comment was
    improved in 4a06e99 ("clarify comment for obsolete git-gnome-keyring",
    2018-09-04).