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.

adding empty-file
adding large-file
adding another-file
removing empty-file
removing large-file
recording removal of large-file as rename to another-file (99% similar)
adding large-file
adding tiny-file
adding small-file
removing large-file
removing tiny-file
recording removal of tiny-file as rename to small-file (82% similar)