Use Ruby Git API in lib/polisher/git/repo.rb (instead of shelling out)

[movitto]

[Fryguy]

As mentioned in #42

The git code in the git directory can be replaced with code that already handles everything you need. I recommend either https://github.com/3ofcoins/minigit , which is a simple shell out to git and does everything similar to how you've already done it, or https://github.com/libgit2/rugged , which is a Ruby binding to libgit2 and is more OO in its approach.

Rugged seems a bit more solid and active than minigit, and as @axilleas mentioned, it's already packaged in Fedora.