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".
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.