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.