rhg: parallellize computation of [unsure_is_modified] [unsure_is_modified] is called for every file for which we can't determine its status based on its size and mtime alone. In particular, this happens if the mtime of the file changes without its contents changing. Parallellizing this improves performance significantly when we have many of these files. Here's an example run (on a repo with ~400k files after dropping FS caches) ``` before: real 0m53.901s user 0m27.806s sys 0m31.325s after: real 0m32.017s user 0m34.277s sys 1m26.250s ``` Another example run (a different FS): ``` before: real 3m28.479s user 0m31.800s sys 0m25.324s after: real 0m29.751s user 0m41.814s sys 1m15.387s ```