subrepo: handle diff with working copy
We cannot unconditionally call bin(node2) since node2 is None when
comparing with the working copy.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.