xmalloc: include size in the failure message

Out-of-memory errors can either be actual lack of memory, or bugs (like
code trying to call xmalloc(-1) by mistake). A little more information
may help tracking bugs reported by users.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

wrapper.c

@@ -40,7 +40,8 @@ void *xmalloc(size_t size)
 		if (!ret && !size)
 			ret = malloc(1);
 		if (!ret)
-			die("Out of memory, malloc failed");
+			die("Out of memory, malloc failed (tried to allocate %lu bytes)",
+			    (unsigned long)size);
 	}
 #ifdef XMALLOC_POISON
 	memset(ret, 0xA5, size);