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

hg init

mkdir alpha
touch alpha/one
mkdir beta
touch beta/two

hg add alpha/one beta/two
hg ci -m "start" -d "1000000 0"

echo 1 > alpha/one
echo 2 > beta/two

echo EVERYTHING
hg diff --nodates

echo BETA ONLY
hg diff --nodates beta

echo INSIDE BETA
cd beta
hg diff --nodates .