Control behavior of rename and copy detection

These options mostly mimic parameters that can be passed to git-diff.

• rename_threshold is the same as the -M option with a value
• copy_threshold is the same as the -C option with a value
• rename_from_rewrite_threshold matches the top of the -B option
• break_rewrite_threshold matches the bottom of the -B option
• rename_limit is the maximum number of matches to consider for a particular file. This is a little different from the -l option to regular Git because we will still process up to this many matches before abandoning the search.

The metric option allows you to plug in a custom similarity metric. Set it to NULL for the default internal metric which is based on sampling hashes of ranges of data in the file. The default metric is a pretty good similarity approximation that should work fairly well for both text and binary data, and is pretty fast with fixed memory overhead.