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
# Test issue 746: renaming files brought by the
# second parent of a merge was broken.
echo % create source repository
hg init t
cd t
echo a > a
hg ci -Am a
cd ..
echo % fork source repository
hg clone t t2
cd t2
echo b > b
hg ci -Am b
echo % update source repository
cd ../t
echo a >> a
hg ci -m a2
echo % merge repositories
hg pull ../t2
hg merge
hg st
echo % rename b as c
hg mv b c
hg st
echo % rename back c as b
hg mv c b
hg st