FilesCacheMixin initialized _newest_cmtime to 0, but _write_files_cache()
only treats None as "no file was chunked this run" (falling back to a
max_time_ns cutoff that keeps all current entries).
When a backup reuses all files from the cache without chunking anything,
_newest_cmtime stayed at 0, so the race-protection cutoff became the unix
epoch and every current (age == 0) entry was discarded. The next backup
then had to re-read, chunk and hash all files again.
Initialize _newest_cmtime to None to match the documented contract in
_write_files_cache(), and make the comparisons in _build_files_cache()
None-safe.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
c08c2ca461