pack-revindex: introduce GIT_TEST_REV_INDEX_DIE_ON_DISK

pack-revindex: introduce GIT_TEST_REV_INDEX_DIE_ON_DISK

In ec8e7760ac (pack-revindex: ensure that on-disk reverse indexes are
given precedence, 2021-01-25), we introduced
GIT_TEST_REV_INDEX_DIE_IN_MEMORY to abort the process when Git generated
a reverse index from scratch.

ec8e7760ac was about ensuring that Git prefers a .rev file when
available over generating the same information in memory from scratch.

In a subsequent patch, we'll introduce `pack.readReverseIndex`, which
may be used to disable reading ".rev" files when available. In order to
ensure that those files are indeed being ignored, introduce an analogous
option to abort the process when Git reads a ".rev" file from disk.

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