wix: switch Mercurial Windows installer to use py2exe --bundle 3 --bundle 3 leaves all of the compiled C extensions and other DLLs outside of the library.zip, so we no longer add the installer folder to the system PATH. Instead, we now ship a small bin/hg.cmd and it is placed in the PATH. Switching to py2exe --bundle 3 is necessary because the higher bundle options are not supported on x64.