Try to avoid a lot of work scanning for excluded files,
by caching some more information when setting up the exclusion
data structure.

Speeds up 'git runstatus' on a repository containing the Qt sources by 30% and
reduces the amount of instructions executed (as measured by valgrind) by a
factor of 2.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>