Upstream git has converted to use `wildmatch` instead of
`fnmatch`. Convert our gitattributes logic to use `wildmatch` as
the last user of `fnmatch`. Please, don't expect I know what I'm
doing here: the fnmatch parser is one of the most fun things to
play around with as it has a sh*tload of weird cases. In all
honesty, I'm simply relying on our tests that are by now rather
comprehensive in that area.

The conversion actually fixes compatibility with how git.git
parser "**" patterns when the given path does not contain any
directory separators. Previously, a pattern "**.foo" erroneously
wouldn't match a file "x.foo", while git.git would match.

Remove the new-unused LEADINGDIR/NOLEADINGDIR flags for
`git_attr_fnmatch`.