gitattributes: Improve highlight.

Add highlight for pattern similar to `gitignore-mode'.  Highlight for
/ is missing (TODO).  Highlight for variables fixed (includes - now,
e.g., for "export-ignore").  Highlight for "[attr]" changed to
`font-lock-function-name-face'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>