Move commit_sha method from GithubFormatter to Rugged::Diff::Line
Add #commit on Rugged::Diff::Line
Monkey-patch a #show method on Rugged::Commit
Extract #blameline method inside Rugged::Diff::Line
Add #commit_line method on Rugged::Diff::Line

Which returns Rugged::Diff::Line from the actual commit diff, not the
whole diff