continue: added logic for hg continue This is part of GSoC19 project `Implement abort and continue commands`. This patch is part of the continue plan. This adds the basic logic for hg continue. This command aborts an multistep operation like graft, histedit, rebase, transplant and unshelve if they are in an unfinished state. The first part of the logic is determining the unfinished operation from the state detection API under statemod. This API is extended to support hg continue by adding a method to register the abort logic as a function (here continuefunc). Once the unfinished operation is determined the registered logic is used to resume the command in case it is interrupted. The benefit of this kind of framework is that any new extension developed can support hg continue by registering the command and logic under statedetection API. hg continue currently supports --dry-run/-n flag only. It is used to dry run hg abort Differential Revision: https://phab.mercurial-scm.org/D6645