Disable "git update" and "git update-git-for-windows" in VFS versions

[derrickstolee]

If a user runs git update-git-for-windows, then they will upgrade to a version that does not support microsoft/vfsforgit or microsoft/scalar.

Find a way to disable this so users do not [up/down]grade in error.

[dscho]

Find a way to disable this so users do not [up/down]grade in error.

I think that's easy: just implement a dummy built-in. I'll try my hand at this, please give me a few minutes.

[dscho]

Closed via #243