Offer a libgit2 backend?

[notatestuser]

Right now we spawn a child process (git) and run a command, capturing stdout to get the results. A better approach might be to make this project a native binding to libgit2, an implementation of git's core functionality in C.

There are several benefits to this kind of approach:

• The module's users would not have to install the git binary on their system
• There's no way to manipulate stuff in memory (do we care?)
• Calling into native code is probably less expensive than starting a process with each method call