parse_attr() only needs access to the attr_state to which it should
store its results, not to the whole match_attr structure. This change
also removes the need for it to know num_attr. Change its signature
accordingly and add a comment.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>