Precommit-diffcheck

* Emit `precommit_diffcheck.DiffcheckError` instead of `UnicodeDecode` exceptions on failure to decode content. Also avoid double-decoding in failure.
* Fix diff command when --from-ref/--to-ref is not provided (default case)

* Show useful error when we can't get a diff due to Unicode issues.
* Improve commands for getting diffs to handle --from-ref better

* Restore support for Python 3.6

* Add `filter_filenames()` and `is_excluded()` functions.
* Add an `lru_cache` on `get_git_root()` for performance.

* Add `get_added_lines_for_file()`
* Add `get_filname_to_added_lines()`

* Normalize filenames when providing file content as a diff