Merge branch 'rs/commit-list-sort-in-batch'

Merge branch 'rs/commit-list-sort-in-batch'

Setting up a revision traversal with many starting points was inefficient
as these were placed in a date-order priority queue one-by-one.

By René Scharfe (3) and Junio C Hamano (1)
* rs/commit-list-sort-in-batch:
  mergesort: rename it to llist_mergesort()
  revision: insert unsorted, then sort in prepare_revision_walk()
  commit: use mergesort() in commit_list_sort_by_date()
  add mergesort() for linked lists