Merge branch 'en/ort-perf-batch-13' into seen

Merge branch 'en/ort-perf-batch-13' into seen

Performance tweaks of "git merge -sort" around lazy fetching of objects.

* en/ort-perf-batch-13:
  merge-ort: add prefetching for content merges
  diffcore-rename: use a different prefetch for basename comparisons
  diffcore-rename: allow different missing_object_cb functions
  t6421: add tests checking for excessive object downloads during merge
  promisor-remote: output trace2 statistics for number of objects fetched