midx: teach `nth_midxed_offset()` about incremental MIDXs

In a similar fashion as in previous commits, teach the function
`nth_midxed_offset()` about incremental MIDXs.

The given object `pos` is used to find the containing MIDX, and
translated back into a MIDX-local position by assigning the return value
of `midx_for_object()` to it.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -369,6 +369,8 @@ off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 	const unsigned char *offset_data;
 	uint32_t offset32;
 
+	pos = midx_for_object(&m, pos);
+
 	offset_data = m->chunk_object_offsets + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH;
 	offset32 = get_be32(offset_data + sizeof(uint32_t));