revision: use priority queue in limit_list()

limit_list() maintains a date-sorted work queue of commits using a linked list with commit_list_insert_by_date() for insertion. Each insertion walks the list to find the right position — O(n) per insert. In repositories with merge-heavy histories, the symmetric difference can contain thousands of commits, making this O(n) insertion the dominant cost. Replace the sorted linked list with a prio_queue (binary heap). This gives O(log n) insertion and O(log n) extraction instead of O(n) insertion and O(1) extraction, which is a net win when the queue is large. The still_interesting() and everybody_uninteresting() helpers are updated to scan the prio_queue's contiguous array instead of walking a linked list. process_parents() already accepts both a commit_list and a prio_queue parameter, so the change in limit_list() simply switches which one is passed. Benchmark: git rev-list --left-right --count HEAD~N...HEAD Repository: 2.3M commits, merge-heavy DAG (monorepo) Best of 5 runs, times in seconds: commits in symmetric diff baseline patched speedup -------------- -------- ------- ------- 10 0.01 0.01 1.0x 50 0.01 0.01 1.0x 3751 21.23 8.49 2.5x 4524 21.70 8.29 2.6x 10130 20.10 6.65 3.0x No change for small traversals; 2.5-3.0x faster when the queue grows to thousands of commits. Signed-off-by: Kristofer Karlsson <krka@spotify.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>