Streaming stdout instead of one response

[aquaflamingo]

Subject of the issue

Hi there, 👋 great library.

For long running git operations, I'm wonder the thoughts on adding a "stream stdout" option to the library instead of the current implementation which takes the bulk response back stores it in a variable and returns it.

Is this something you think would be relatively simple and / or desirable?

[jcouball]

#617 should enable doing this. It isn't possible with the current way that the library calls git with backticks.

[jcouball]

Fixed with #684 and released in git-2.0.0-pre1