revlog: efficient implementation of 'descendant' Iterating over descendants is costly, because there are no "parent -> children" pointers. Walking the other way around is much more efficient, especially on large repositories, where descendant walks can cost seconds. And the other hand, common ancestors code follows links in the right direction and has a compiled implementation. In real life usage, this saved up to 80s during some pull operations, where descendant test happens in extension code.

revlog: refactor out the rev-oriented part of commonancestorheads We plan to use this in a function taking revs as argument. Round trips to nodes seem silly.

revlog: do inclusive descendant testing (API) In many other places, a revision is considered a descendant of itself. We update the behavior of `revlog.descendant()` to match this. (for example. `revlog.isancestor` does inclusive testing). No tests break, so it seems safe to do so. This will make it easier to use a more efficient implementation in a later changeset.

manifest: make cachesize a private attribute AFAICT this isn't accessed outside the class. It is a private attribute and its naming should reflect that. Differential Revision: https://phab.mercurial-scm.org/D3868

repository: document that file() return value conforms to interface Differential Revision: https://phab.mercurial-scm.org/D3867

cleanup: pass in overwrite flag to hg.updaterepo() as named argument For clarity.