Increment num_attr in parse_attr_line(), not parse_attr()

num_attr is incremented iff parse_attr() returns non-NULL.  So do the
counting in parse_attr_line() instead of within parse_attr().  This
allows an integer rather than a pointer to an integer to be passed to
parse_attr().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>