t/t7700-repack.sh: fix test breakages with `GIT_TEST_MULTI_PACK_INDEX=1 `

t/t7700-repack.sh: fix test breakages with `GIT_TEST_MULTI_PACK_INDEX=1 `

There are a handful of related test breakages which are found when
running t/t7700-repack.sh with GIT_TEST_MULTI_PACK_INDEX set to "1" in
your environment.

Both test failures are the result of something like:

    git repack --write-midx --write-bitmap-index [...] &&

    test_path_is_file $midx &&
    test_path_is_file $midx-$(midx_checksum $objdir).bitmap

, where we repack instructing Git to write a new MIDX and corresponding
MIDX bitamp.

The error occurs when GIT_TEST_MULTI_PACK_INDEX=1 is found in the
enviornment. This causes Git to write out a second MIDX (after
processing the builtin's `--write-midx` argument) which is identical to
the first, but does not request a bitmap (since we did not set the
GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP variable in the environment).

Since c528e179662 (pack-bitmap: write multi-pack bitmaps, 2021-08-31),
the MIDX machinery will drop an existing MIDX bitmap when rewriting an
identical MIDX which does not itself request a corresponding bitmap,
which is similar to the way repack itself behaves in the pack-bitmap
case.

Correct these issues (which date back to [1] and [2], respectively) by
explicitly setting GIT_TEST_MULTI_PACK_INDEX to zero before running each
command.

In the future, we should consider removing GIT_TEST_MULTI_PACK_INDEX,
and in general clean up unused GIT_TEST_-variables. But that is a larger
effort, and this ensures that we can cleanly run:

    $ GIT_TEST_MULTI_PACK_INDEX=1 make test

in the meantime.

[1]: 324efc90d1b (builtin/repack.c: pass `--refs-snapshot` when writing
  bitmaps, 2021-10-01)

[2]: 197443e80ab (repack: don't remove .keep packs with
  `--pack-kept-objects`, 2022-10-17).

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