abort: added logic for of hg abort This is part of `GSoC19` project `Implement abort and continue commands`. This patch is part of the `abort plan`. This adds the basic logic for `hg abort`. This command aborts an multistep operation like graft, histedit, rebase, merge 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 abort` by adding a method to register the abort logic as a function (here `abortfunc`). Once the unfinished operation is determined the registered logic is used to abort the command. The benefit of this kind of framework is that any new extension developed can support `hg abort` by registering the command and logic under statedetection API. `hg abort` currently supports `--dry-run/-n` flag only. It is used to dry run `hg abort` Further patches sequentially add support for `graft`, `rebase`, `unshelve`, `histedit` and `merge`. Differential Revision: https://phab.mercurial-scm.org/D6566