Implemented parameters for Diff.find_similar()

Diff.find_similar now supports the parameters that are exposed by libgit2's git_diff_find_similar.

Parameters supported are rename_threshold, copy_threshold,
rename_from_rewrite_threshold, break_rewrite_threshold and rename_limit
All of these values will be unitialised if they're not passed. Instead of this, you should let `PyArg_ParseTupleAndKeywords` write into the options structure directly.