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
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a       
echo a > a 
hg commit -Ama 

hg parents -r 0 doesnotexist
true