util._matcher: speed up regexp matching.
In 4babaa52badf, Benoit made a change that substantially slows matching
when a big .hgignore file is in play, because it calls into the regexp
matching engine potentially hundreds of times per file to be matched.
I've partly rolled back his change, so that we only call into the matcher
once per file, but preserved the ability to report a meaningful error
message if there's a syntax error in the regexp.
% init foo-base
% create alpha in first repo
adding alpha
% clone foo-base to foo-work
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% create beta in second repo
adding beta
% create gamma in first repo
adding gamma
% pull into work and merge
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
% revert to changeset 1 to simulate a failed merge
2 files updated, 0 files merged, 0 files removed, 0 files unresolved