t7900: clean up some more broken refs

t7900: clean up some more broken refs

The "incremental-repack task" test replaces the object directory with a
known state. As a result, some of our refs point to objects that are not
included in that state.

Commit 3cf5f221be (t7900: clean up some broken refs, 2021-01-19) cleaned
up some of those (that were causing warnings to stderr from the
maintenance process). But there are a few more that were missed. These
aren't hurting anything for now, but it's certainly an unexpected state
to leave the test repository in, and it will become a problem if repack
ever gets more picky about broken refs.

Let's clean up those additional refs (which are all in refs/remotes,
with nothing there that isn't broken), and add an extra "for-each-ref"
call to assert that we've got everything.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>