mergesort: rename it to llist_mergesort()

mergesort: rename it to llist_mergesort()

Even though the function is generic enough, <anything>sort() inherits
connotations from the standard function qsort() that sorts an array.
Rename it to llist_mergesort() and describe the external interface in
its header file.

This incidentally avoids name clashes with mergesort() some platforms
declare in, and contaminate user namespace with, their <stdlib.h>.

Reported-by: Brian Gernhardt
Signed-off-by: Junio C Hamano <redacted>