dirstate: make dir collision logic faster
- shortcircuit decpath if we haven't built the _dirs map
- increment only for leafnodes of directory tree
(this should make construction more like O(nlog n) than O(n^2))
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.