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>

test-mergesort.c

@@ -42,7 +42,7 @@ int main(int argc, const char **argv)
 		p = line;
 	}
 
-	lines = mergesort(lines, get_next, set_next, compare_strings);
+	lines = llist_mergesort(lines, get_next, set_next, compare_strings);
 
 	while (lines) {
 		printf("%s", lines->text);