revlog: add rev-specific variant of findmissing This will be used by rebase in an upcoming commit.

revlog: switch findmissing to use ancestor.missingancestors This also speeds up other commands that use findmissing, like incoming and merge --preview. With a large linear repository (>400000 commits) and with one incoming changeset, incoming is sped up from around 4-4.5 seconds to under 3.

ancestor: faster algorithm for difference of ancestor sets One of the major reasons rebase is slow in large repositories is the computation of the detach set: the set of ancestors of the changesets to rebase not in the destination parent. This is currently done via a revset that does two walks all the way to the root of the DAG. Instead of doing that, to find ancestors of a set <revs> not in another set <common> we walk up the tree in reverse revision number order, maintaining sets of nodes visited from <revs>, <common> or both. For the common case where the sets are close both topologically and in revision number (relative to repository size), this has been found to speed up rebase by around 15-20%. When the nodes are farther apart and the DAG is highly branching, it is harder to say which would win. Here's how long computing the detach set takes in a linear repository with over 400000 changesets, rebasing near tip: Rebasing across 4 changesets Revset method: 2.2s New algorithm: 0.00015s Rebasing across 250 changesets Revset method: 2.2s New algorithm: 0.00069s Rebasing across 10000 changesets Revset method: 2.4s New algorithm: 0.019s