dir.c: refactor is_excluded()

dir.c: refactor is_excluded()

In a similar way to the previous commit, this extracts a new helper
function last_exclude_matching() which returns the last exclude_list
element which matched, or NULL if no match was found.  is_excluded()
becomes a wrapper around this, and just returns 0 or 1 depending on
whether any matching exclude_list element was found.

This allows callers to find out _why_ a given path was excluded,
rather than just whether it was or not, paving the way for a new git
sub-command which allows users to test their exclude lists from the
command line.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>