midx: teach `midx_contains_pack()` about incremental MIDXs

midx: teach `midx_contains_pack()` about incremental MIDXs

Now that the `midx_contains_pack()` versus `midx_locate_pack()` debacle
has been cleaned up, teach the former about how to operate in an
incremental MIDX-aware world in a similar fashion as in previous
commits.

Instead of using either of the two `midx_for_object()` or
`midx_for_pack()` helpers, this function is split into two: one that
determines whether a pack is contained in a single MIDX, and another
which calls the former in a loop over all MIDXs.

This approach does not require that we change any of the implementation
in what is now `midx_contains_pack_1()` as it still operates over a
single MIDX.

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