Diff Buffers

This exposes the new `git_patch_from_buffers` as `Rugged::Diff.diff_buffers`.

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@brianmario What do you think about changing the name to `Rugged::Patch.from_strings`? There is no concept of "buffers" in Ruby, and this actually returns a `Rugged::Patch` and not a `Rugged::Diff`.