setup.py: attempt to build and install hg.exe on Windows Currently, packaging Mercurial on Windows will produce a Scripts\hg Python script and a Scripts\hg.bat batch script. The py2exe distribution contains a hg.exe which loads a Python interpretter and invokes the "hg" Python script. Running a exe directly has benefits over batch scripts because batch scripts do things like muck around with command arguments. This patch implements a custom "build_scripts" command which attempts to build hg.exe on Windows. If hg.exe is built, it is marked as a "script" file and installed into the Scripts\ directory on Windows. Since hg.exe is redundant and better than hg.bat, if hg.exe is built, hg.bat is not installed. Since some environments don't support compiling C programs, we treat hg.exe as optional and catch failures building it. This is not ideal. However, I reckon most Windows users will not be installing Mercurial from source: they will get it from the MSI installer or via `pip install Mercurial`, which will download a wheel that has hg.exe in it. So, I don't think this is a big deal.