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
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
Results are shown as tests.
Differential Revision: https://phab.mercurial-scm.org/D6579