ancestor.missingancestors: don't discard from basesvisit
We only actually care about whether revsvisit is empty, so we can let
basesvisit grow to arbitrary size.
It turns out that this actually helps performance. For a large repo with
hundreds of thousands of commits, hg perfrevset 'only(0, tip)' (basically the
worst case, involving a full DAG traversal) goes from 1.63 seconds to 1.50. hg
perfrevset 'only(tip, 0)' remains unchanged at 1.98 seconds.