Merge branch 'kk/streaming-walk-pqueue' into jch

Merge branch 'kk/streaming-walk-pqueue' into jch

Streaming revision walks have been optimized by using a priority queue
for date-sorting commits, speeding up walks repositories with many
merges.

* kk/streaming-walk-pqueue:
  revision: use priority queue for non-limited streaming walks
  revision: introduce rev_walk_mode to clarify get_revision_1()
  pack-objects: call release_revisions() after cruft traversal