midx-write.c: extract `should_include_pack()`

The add_pack_to_midx() callback used via for_each_file_in_pack_dir() is
used to add packs with .idx files to the MIDX being written.

Within this function, we have a pair of checks that discards packs
which:

  - appear in an existing MIDX, if we successfully read an existing MIDX
    from disk

  - or, appear in the "to_include" list, if invoking the MIDX write
    machinery with the `--stdin-packs` command-line argument.

A future commit will want to call a slight variant of these checks from
the code that reuses all packs from an existing MIDX, as well as the
current location via add_pack_to_midx(). The latter will be modified in
subsequent commits to only reuse packs which appear in the to_include
list, if one was given.

Prepare for that step by extracting these checks as a subroutine that
may be called from both places.

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

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