Merge branch 'rs/mergesort'

Merge branch 'rs/mergesort'

Make our mergesort implementation type-safe.

* rs/mergesort:
  mergesort: remove llist_mergesort()
  packfile: use DEFINE_LIST_SORT
  fetch-pack: use DEFINE_LIST_SORT
  commit: use DEFINE_LIST_SORT
  blame: use DEFINE_LIST_SORT
  test-mergesort: use DEFINE_LIST_SORT
  test-mergesort: use DEFINE_LIST_SORT_DEBUG
  mergesort: add macros for typed sort of linked lists
  mergesort: tighten merge loop
  mergesort: unify ranks loops