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.

changeset:   0:0acdaf898367
tag:         tip
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     test

b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 644 a
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