gitattributes: Add forward-field operation.

Sets `forward-sexp-function' to `gitattributes-mode-forward-field' to
treat fields as "sexps".  Also add a menu.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
gitattributes: Add function to open manpage.

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