mergesort: avoid left shift overflow

Use size_t to match n when building the bitmask for checking whether a
rank is occupied, instead of the default signed int.

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

mergesort.c

@@ -63,7 +63,7 @@ void *llist_mergesort(void *list,
 		void *next = get_next_fn(list);
 		if (next)
 			set_next_fn(list, NULL);
-		for (i = 0; n & (1 << i); i++)
+		for (i = 0; n & ((size_t)1 << i); i++)
 			list = llist_merge(ranks[i], list, get_next_fn,
 					   set_next_fn, compare_fn);
 		n++;