Make it work on Windows

[XhmikosR]

Not sure how much work will this be, but currently this doesn't work on Windows.

I'll add more info later.

[XhmikosR]

@rvagg it seems the failures on Windows are coming from gitexec. I have a WIP branch, which works but I'm not 100% sure it's safe.

So, perhaps, we could switch to something else like cross-spawn?

[DerekNonGeneric]

@XhmikosR, please let me know what the issue is on this.

I do have a Windows computer to test on at the moment.