windows: add an experimental option for long paths support
This commit adds an experimental --long-paths-support flag to build_hgexe
on Windows. It is off by default, but when supplied, causes setup.py to
embed some XML into the generated hg.exe, which in turn tells Windows to
allow this exe to use long paths (given that the appropriate registry setting
is enabled as well).
This was tested on Windows 10 14393 and 15063.
This commit introduces a badly-named initialize_options function, but its name
is dictated by distutils, rather than chosen.