t5326: extract `test_rev_exists`

t5326: extract `test_rev_exists`

To determine which source of data is used for the MIDX's reverse index
cache, introduce a helper which forces loading the reverse index, and
then looks for the special trace2 event introduced in a previous commit.

For now, this helper just looks for when the legacy MIDX .rev file was
loaded, but in a subsequent commit will become parameterized over the
the reverse index's source.

This function replaces checking for the existence of the .rev file. We
could write a similar helper to ensure that the .rev file is cleaned up
after repacking, but it will make subsequent tests more difficult to
write, and provides marginal value since we already check that the MIDX
.bitmap file is removed.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>