Rename detection using diff.renames

As noticed in https://github.com/libgit2/libgit2sharp/pull/573#issuecomment-29854242, there's an unsupported use-case with `git_diff_find_similar`, where the caller wants to obey the `diff.renames` config value for how hard to try, but possibly override some or all of the threshold values (i.e. `git diff -B70%`).

- [x] Add new similarity configuration flag
- [x] Implement the right behavior
- [x] Decide whether it should be the default
- [x] Tests