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

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status