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 <gitster@pobox.com>

mergesort.c

@@ -23,10 +23,10 @@ static void *pop_item(struct mergesort_sublist *l,
 	return p;
 }
 
-void *mergesort(void *list,
-		void *(*get_next_fn)(const void *),
-		void (*set_next_fn)(void *, void *),
-		int (*compare_fn)(const void *, const void *))
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *))
 {
 	unsigned long l;