convert: use a priority queue for sorting commits, to make sorting faster To achieve this, we turn commit sorters into classes so they can encapsulate state. This reduces the sorting time from ~30s to ~10s on a 500k-commit prefix of a repo I tried to convert. (and probably reduces the time to sort the whole repo from many tens of minutes to minutes, but I didn't try that again) The date caching gets removed because priority queue already caches the key.

dirstate-v2: don't mmap the data file when on NFS `mmap` on NFS will trigger a SIGBUS when the mmap'ed file is deleted, which wouldn't work in our case. Also, the performance advantage of using mmap on NFS is debatable at best.