Use git bindings library

[stellarhoof]

Right now autoFix calls the git cmdline as an external process directly,
which is a very ad-hoc approach. More robust would be to use js bindings
to libgit (https://github.com/nodegit/nodegit).

Another advantage of using a library for interacting with git is that we can
mock the library, whereas right now it's very hard to test current functionality.
We'd potentially have to mock the child_process node module itself and
parse the command to verify the right processes with the right arguments
are being called.