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.