performance: do not stat() things when not required
Ignore unknown files if we don't need them (eg in hg diff).
It slows things down a little bit for big trees (kernel repo), since _join()
is called for each file instead of for each directory.
fix issue567
#!/bin/sh
mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"
chmod -w .hg/store
cd ..
hg clone a b
chmod +w a/.hg/store # let test clean up
cd b
hg verify