test-mergesort: add test subcommand

test-mergesort: add test subcommand

Adapt the qsort certification program from "Engineering a Sort Function"
by Bentley and McIlroy for testing our linked list sort function.  It
generates several lists with various distribution patterns and counts
the number of operations llist_mergesort() needs to order them.  It
compares the result to the output of a trusted sort function (qsort(1))
and also checks if the sort is stable.

Also add a test script that makes use of the new subcommand.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>