dirstate: make parents() faster. This allows localrepository.status() to avoid reading all but the first 40 bytes of the dirstate in the common case of running "hg status".

#!/bin/sh

hg init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg/store
hg --cwd b push ../a
chmod 700 a/.hg/store