abort: added support for unshelve This patch adds the support for shelve in `hg abort` plan. For this the logic to load a `shelvedstate` and the error handling for it had been shifted to a seperate function `_loadunshelvedstate()`. This returns a tuple with `state` file and `opts.` `hgabortunshelve()` has been created for independent calls. In case abortion of `unshelve` is called via `hg abort` the `shelvedstate` needs to be loaded seperately. This has been ensured by `_loadunshelvedstate()` `hgabortunshelve()` is then registered as `abortfunc` for state detection API. Results are shown as tests. Differential Revision: https://phab.mercurial-scm.org/D6579