t/helper/test-read-midx.c: add --checksum mode
Subsequent tests will want to check for the existence of a multi-pack
bitmap which matches the multi-pack-index stored in the pack directory.
The multi-pack bitmap includes the hex checksum of the MIDX it
corresponds to in its filename (for example,
'$packdir/multi-pack-index-<checksum>.bitmap'). As a result, some tests
want a way to learn what '<checksum>' is.
This helper addresses that need by printing the checksum of the
repository's multi-pack-index.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>