Package git in the app

[joshaber]

Dependent on #5.

We'll need to shell out to git sometimes, so it'll need to be in the app.

• macOS
• Windows

[shiftkey]

We've had a bunch of discussion recently about whether we can live without the entire Git for Windows package in Desktop - @dscho told me about his experiment with slimming down the dependencies context here - so I'll have a look into whether we can make the Windows app behave similar to the Mac app.

[joshaber]

Putting this in Bedrock since we'll need it to do most things with git.

[joshaber]

I'll do this macOS side of this.