Move to Rugged

i would like to see gollum working on Windows and see what great features and fixes can come about because of Rugged.

This is moved over from my old pull: gollum/gollum#617. i've taken some steps back from that PR but i still want to put this out there to invite help and to see builds for this PR break like crazy :collision: 

## Roadblocks

Two things need to happen with the Rugged library before this pull can be finished.

- [x] Diff support (libgit2/Rugged#181)
- [x] Checkout support so gollum can work in a working directory

## Tests

Tests that need to get passed:

- [ ] Committer
- [ ] File
- [x] File View
- [x] Git Access
- [ ] Markup
- [ ] Page
- [ ] Page Revert
- [ ] Remote code
- [ ] Unicode
- [ ] Wiki