dir.c: do not be fooled by :(exclude) pathspec elements

dir.c: do not be fooled by :(exclude) pathspec elements

When exclude_matches_pathspec() tries to determine if an otherwise
excluded item matches the pathspec given, it goes through each
pathspec element and declares a hit, without checking if the element
is a negative ":(exclude)" element.  Fix it be applying the usual "a
path matches if it matches any one of positive pathspec element, and
if it matches none of negative pathspec elements" rule in the
function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>