HashIndex: in cleanup, interpret missing dir as completed merge

If we stop between unlinking the empty subdir and removing the root
merge marker, we get ENOENT on the get_info.  That's actually fine.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'upstream/wip-8104', 'upstream/wip-8334', 'upstream/wip-8338' and 'upstream/wip-8332' into wip-sam-testing