fedpkg: Try not to add redundant gitignore entries
    
    If there is a entry which matches the filename we're about to add
    already in the gitignore file, we don't need to bother adding another
    entry.
    
    This adds a match() method to GitIgnore which uses fnmatch.  Under the
    hood, this is what git uses.  It doesn't catch some of the special cases
    git has, but it should simply fail to match in those cases and will add
    a potentially redundant entry -- no different than previously.