Message 275671 - Python tracker

Here is a patch to add sys._git and update Py_GetBuildInfo() as necessary. I would like to see this in Python 3.6 as 3.6.0 will be released using Mercurial but (hopefully) subsequent releases will be on git, so any reviews will be appreciated.

I will hold off on removing sys._mercurial until 3.7 as that will be the first release that doesn't span version control systems.