ancestor: a new algorithm that is faster for nodes near tip Instead of walking all the way to the root of the DAG, we generate a set of candidate GCA revs, then figure out which ones will win the race to the root (usually without needing to traverse all the way to the root). In the common case of nodes that are close to each other in both revision number and topology, this is usually a big win: it makes "hg --time debugancestors" up to 9 times faster than the more general ancestor function when measured on heads of the linux-2.6 hg repo. Victory is not assured, however. The older function can still win by a large margin if one node is much closer to the root than the other, or by a much smaller amount if one is an ancestor of the other. For now, we've also got a small paranoid harness function that calls both ancestor functions on every input and ensures that they give equivalent answers. Even without the checker function, the old ancestor function needs to stay alive for the time being, as its generality is used by context.filectx.merge.