The infinitepush backup smartlog summary uses only(a, b) to determine which
revisions have not been backed up. However, if the set of backup heads is
empty, only() behaves as if it were called as only(a), which is wrong and slow.
Instead, use ::a - ::b, which is always correct, even if b is empty.
This is expanded to (draft() and ::%ls) - ::%ls.