pack-revindex.c: instrument loading on-disk reverse index

pack-revindex.c: instrument loading on-disk reverse index

In a subsequent commit, we'll use the MIDX's new 'RIDX' chunk as a
source for the reverse index's data. But it will be useful for tests to
be able to determine whether the reverse index was loaded from the
separate .rev file, or from a chunk within the MIDX.

To instrument this, add a trace2 event which the tests can look for in
order to determine the reverse index's source.

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>