t5325: check both on-disk and in-memory reverse index

t5325: check both on-disk and in-memory reverse index

Right now, the test suite can be run with 'GIT_TEST_WRITE_REV_INDEX=1'
in the environment, which causes all operations which write a pack to
also write a .rev file.

To prepare for when that eventually becomes the default, we should
continue to test the in-memory reverse index, too, in order to avoid
losing existing coverage. Unfortunately, explicit existing coverage is
rather sparse, so only a basic test is added that compares the result of

    git rev-list --objects --no-object-names --all |
    git cat-file --batch-check='%(objectsize:disk) %(objectname)'

with and without an on-disk reverse index.

Suggested-by: Jeff King <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>