t7700: check post-condition in kept-pack test

t7700: check post-condition in kept-pack test

The '--write-midx -b packs non-kept objects' test in t7700-repack.sh
uses test_subcommand_inexact to check that 'git repack' properly adds
the '--honor-pack-keep' flag to the 'git pack-objects' subcommand.
However, the test_subcommand_inexact helper is more flexible than
initially designed, and this instance is the only one that makes use of
it: there are additional arguments between 'git pack-objects' and the
'--honor-pack-keep' flag. In order to make test_subcommand_inexact more
strict, we need to fix this instance.

This test checks that 'git repack --write-midx -a -b -d' will create a
new pack-file that does not contain the objects within the kept pack.
This behavior is possible because of the multi-pack-index bitmap that
will bitmap objects against multiple packs. Without --write-midx, the
objects in the kept pack would be duplicated so the resulting pack is
closed under reachability and bitmaps can be created against it. This is
discussed in more detail in e4d0c11c0 (repack: respect kept objects with
'--write-midx -b', 2021-12-20) which also introduced this instance of
test_subcommand_inexact.

To better verify the intended post-conditions while also removing this
instance of test_subcommand_inexact, rewrite the test to check the list
of packed objects in the kept pack and the list of the objects in the
newly-repacked pack-file _other_ than the kept pack. These lists should
be disjoint.

Be sure to include a non-kept pack-file and loose objects to be extra
careful that this is properly behaving with kept packs and not just
avoiding repacking all pack-files.

Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>