But isn't that just Bash and a few other tools here and there ?
This is a more complete suit of tools that intends to be sufficient for building any Gnu/Linux/Posix software on a Windows platform. A much more ambitious goal.
Git-4-Windows is a great project, just with a different focus. They contribute a lot to our packages (and Cygwin) too since git (and the shell) drags in quite a lot of stuff.
We need to merge our work carefully. Git-4-Windows with pacman and makepkg wouldn't work too well at present, as MSYS2 with a native git wouldn't work too well at present.