.arcconfig

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

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