t: prepare for GIT_TEST_WRITE_REV_INDEX

t: prepare for GIT_TEST_WRITE_REV_INDEX

In the next patch, we'll add support for unconditionally enabling the
'pack.writeReverseIndex' setting with a new GIT_TEST_WRITE_REV_INDEX
environment variable.

This causes a little bit of fallout with tests that, for example,
compare the list of files in the pack directory being unprepared to see
.rev files in its output.

Those locations can be cleaned up to look for specific file extensions,
rather than take everything in the pack directory (for instance) and
then grep out unwanted items.

Once the pack.writeReverseIndex option has been thoroughly
tested, we will default it to 'true', removing GIT_TEST_WRITE_REV_INDEX,
and making it possible to revert this patch.

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