tortoisehg: 3.7.1 -> 3.7.3, refactor

update tortoisehg to 3.7.3 and remove source patch in build expression.
setup.py in TortoiseHG computes wrong paths in `build` phase
(upstream bug https://bitbucket.org/tortoisehg/thg/issues/4483/problems-with-setuppy-on-posix-systems).
So we skip this phase and perform install manually (the `build` phase is still performed as subcommand).