This is partly to prepare for merging the operations in order of
transaction-commit time (currently merged in order of operation id),
so we can get a predictable order in tests (assuming transactions are
not committed the same millisecond).
`Transaction::add_head()` and others would let the caller add
non-heads to the set (i.e. ancestors of others heads) and the the
non-heads were filterd out when the transaction was committed. That's
a little surprising, so let's try to keep the set valid even within a
transaction. That will surely make commands that add many commits
noticeably slower in large repos. Hopefully we can improve that
later.
