wix: new GUID for contrib.guid
to comply with the component GUID rules of Windows Installer, applied to
the change 6d9ff3374a81
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.