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.
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "mq=" >> $HGRCPATH
hg init a
cd a
echo 'base' > base
hg ci -Ambase -d '1 0'
hg qnew -mmqbase mqbase
hg qrename mqbase renamed
mkdir .hg/patches/foo
hg qrename renamed foo
hg qseries
ls .hg/patches/foo
mkdir .hg/patches/bar
hg qrename foo/renamed bar
hg qseries
ls .hg/patches/bar
hg qrename bar/renamed baz
hg qseries
ls .hg/patches/baz