Before there was a `getcwd` syscall for every filename which was filtered.
Instead this is now cached per-run.
- When all files are identified by filename only: ~45% improvement
- When no files are identified by filename only: ~55% improvement
This makes little difference to overall execution, the bigger win is
eliminating the `memoize_by_cwd` hack. Just removing the memoization would
have *increased* the runtime by 300-500%.
