Fix tuple-chain-moving tests to handle marked-for-update tuples correctly

(they are not part of a chain).  When failing to find a parent tuple in
an update chain, emit a warning and abandon repair_frag, but do not give
an error as before.  This should eliminate the infamous 'No one parent tuple
was found' failure, which we now realize is not a can't-happen condition
but a perfectly valid database state.  Per recent pghackers discussion.