midx-write.c: support reading an existing MIDX with `packs_to_include`

midx-write.c: support reading an existing MIDX with `packs_to_include`

Avoid unconditionally copying all packs from an existing MIDX into a new
MIDX by checking that packs added via `fill_packs_from_midx()` don't
appear in the `to_include` set, if one was provided.

Do so by calling `should_include_pack()` from both `add_pack_to_midx()`
and `fill_packs_from_midx()`.

In order to make this work, teach `should_include_pack()` a new
"exclude_from_midx" parameter, which allows skipping the first check.
This is done so that the caller in `fill_packs_from_midx()` doesn't
reject all of the packs it provided since they appear in an existing
MIDX by definition.

The sum total of this change is that we are now able to read and
reference objects in an existing MIDX even when given a non-NULL
`packs_to_include`. This is a prerequisite step for incremental MIDXs,
which need to load any existing MIDX (if one is present) in order to
determine whether or not an object already appears in an earlier portion
of the MIDX to avoid duplicating it across multiple portions.

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