unshelve: clear shelvedstate and _finishunshelve() on partial unshelve On a partial unshelve, `shelvedstate` was not cleared and `_finishunshelve()` was not called. Ideally, these should be called on this case. This patch makes `shelvedstate` to delete after a successful partial unshelve and calls `_finishunshelve()` in the same case. Differential Revision: https://phab.mercurial-scm.org/D6708