Switch to using a go git package

[Cryptophobia]

From @Joshua-Anderson on September 7, 2016 23:3

Right now we're shelling out to the git command line client to manage git remotes. This is finicky and git version dependent.

Ideally we would use a library so we don't have to use our picky git output parsing code.

This best option right now is https://github.com/libgit2/git2go However, it's based on libgit2, a C library. This removes the option of cross compiling and adds 5 MB to the binary, making it a non-option right now.

I don't think this is anything we can fix soon, I intend this issue mostly to highlight technical debt.

Copied from original issue: deis/workflow-cli#224