Merge branch 'rs/mergesort'

The mergesort implementation used to sort linked list has been
optimized.

* rs/mergesort:
  test-mergesort: use repeatable random numbers
  mergesort: use ranks stack
  p0071: test performance of llist_mergesort()
  p0071: measure sorting of already sorted and reversed files
  test-mergesort: add unriffle_skewed mode
  test-mergesort: add unriffle mode
  test-mergesort: add generate subcommand
  test-mergesort: add test subcommand
  test-mergesort: add sort subcommand
  test-mergesort: use strbuf_getline()