continue: added support for unshelve
This patch adds the support for ushelve in hg continue plan.
hgcontinueunshelve() has been created for independent calls.
In case an interrupted unshelve is resumed via hg continue the
shelvedstate needs to be loaded seperately. This has been
ensured by _loadunshelvedstate()
hgcontinueunshelve() is then registered as continuefunc for state
Results are shown as tests.
Differential Revision: https://phab.mercurial-scm.org/D6652