Implementing file get version #8

THe problem while getting version is this:

- If you're working on a file and it's dirty, you will get a merge exception
- If you're working on a file and we force it you will loose your work
- when you checkout and force a commit the file will be modified and staged
(last one probably libgGit2Sharp)

Going forward we'd like to

- treat merging conflicts with dirty index, maybe pulling merge form for instance
- Let the user decide if he wants to force it or not