context: explicitly take diffopts in `context.diff` (API) To provide a proper replacement for the `patch.diff(…)` function, the `context.diff(…)` method needs to be able to take more parameters. To distinguish the diff options from the new other arguments, we upgrade the diff options to its own explicit argument.

rebase: check whether the rebasestate exists or not a bit early Converted the else part into `if True` because that part will soon be under a except part. Differential Revision: https://phab.mercurial-scm.org/D3876

rebase: use staeobj to check whether interrupted rebase exists This cleans up the code a bit. Differential Revision: https://phab.mercurial-scm.org/D3875

rebase: add a stateobj variable to rebaseruntime class The stateobj variable is an object of state.cmdstate() and will be used in upcoming patches to clean up the logic a bit and also use cbor format to write data to rebasestate class. Differential Revision: https://phab.mercurial-scm.org/D3874

rebase: refactor logic to read rebasestate in a separate function This will help us in plugging the use of state.cmdstate() to read rebasestate. Differential Revision: https://phab.mercurial-scm.org/D3873

revlog: reuse 'descendant' implemention in 'isancestor' The two functions do the same thing, but one takes nodes while the other takes revs. Using one to implement the other make sense. We should probably cleanup the API at some point to avoid having so many similar functions. However, we focus on an efficient implementation for now.

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