dirstate.invalidate: avoid rebuilding _map
Since hasattr will call __getattr__, the call to hasattr(self, '_dirs')
will end up reparsing the dirstate file.
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.