delta-find: add debug information about reuse of cached data This will help us to understand the behavior of find-delta during a pull.

cffi: fix a bytes vs str issue on macOS when listing directories This code hasn't been touched in recent years, and the other implementation return bytes for the filename, so I assume this is a holdover from the py2 days. I was unable to test it on mac though, because the `_osutil` import failed.