Merge branch 'jk/fetch-pack-many-refs'

Fetching between repositories with many refs employed O(n^2)
algorithm to match up the common objects, which has been corrected.

* jk/fetch-pack-many-refs:
  fetch-pack: avoid quadratic behavior in rev_list_push
  commit.c: make compare_commits_by_commit_date global
  fetch-pack: avoid quadratic list insertion in mark_complete