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.h

@@ -1,9 +1,17 @@
 #ifndef MERGESORT_H
 #define MERGESORT_H
 
-void *mergesort(void *list,
-		void *(*get_next_fn)(const void *),
-		void (*set_next_fn)(void *, void *),
-		int (*compare_fn)(const void *, const void *));
+/*
+ * Sort linked list in place.
+ * - get_next_fn() returns the next element given an element of a linked list.
+ * - set_next_fn() takes two elements A and B, and makes B the "next" element
+ *   of A on the list.
+ * - compare_fn() takes two elements A and B, and returns negative, 0, positive
+ *   as the same sign as "subtracting" B from A.
+ */
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *));
 
 #endif