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