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.

test-ancestor: use random testing for missing ancestors We're going to make changes to the missing ancestor algorithm, and random testing will give us much more confidence than a fixed set of tests.