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 <peff@peff.net> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>