argv-array: fix bogus cast when freeing array

Since the array struct stores a "const char **" argv member
(for compatibility with most of our argv-taking functions),
we have to cast away the const-ness when freeing its
elements.

However, we used the wrong type when doing so.  It doesn't
make a difference since free() take a void pointer anyway,
but it can be slightly confusing to a reader.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

argv-array.c

@@ -63,7 +63,7 @@ void argv_array_clear(struct argv_array *array)
 	if (array->argv != empty_argv) {
 		int i;
 		for (i = 0; i < array->argc; i++)
-			free((char **)array->argv[i]);
+			free((char *)array->argv[i]);
 		free(array->argv);
 	}
 	argv_array_init(array);