midx: read `RIDX` chunk when present

When a MIDX contains the new `RIDX` chunk, ensure that the reverse index is read from it instead of the on-disk .rev file. Since we need to encode the object order in the MIDX itself for correctness reasons, there is no point in storing the same data again outside of the MIDX. So, this patch stops writing separate .rev files, and reads it out of the MIDX itself. This is possible to do with relatively little new code, since the format of the RIDX chunk is identical to the data in the .rev file. In other words, we can implement this by pointing the `revindex_data` field at the reverse index chunk of the MIDX instead of the .rev file without any other changes. Note that we have two knobs that are adjusted for the new tests: GIT_TEST_MIDX_WRITE_REV and GIT_TEST_MIDX_READ_RIDX. The former controls whether the MIDX .rev is written at all, and the latter controls whether we read the MIDX's RIDX chunk. Both are necessary to ensure that the test added at the beginning of this series continues to work. This is because we always need to write the RIDX chunk in the MIDX in order to change its checksum, but we want to make sure reading the existing .rev file still works (since the RIDX chunk takes precedence by default). Arguably this isn't a very interesting mode to test, because the precedence rules mean that we'll always read the RIDX chunk over the .rev file. But it makes it impossible for a user to induce corruption in their repository by adjusting the test knobs (since if we had an either/or knob they could stop writing the RIDX chunk, allowing them to tweak the MIDX's object order without changing its checksum). 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>