shelve: restore unshelved dirstate explicitly after aborting transaction

Before this patch, "hg unshelve" uses aborting a current transaction
to discard temporary changes while unshelving.

This assumes that dirstate changes in a transaction scope are kept
even after aborting it. But this assumption will be broken by
"transactional dirstate". See the wiki page below for detail about it.

    https://mercurial.selenic.com/wiki/DirstateTransactionPlan

This patch explicitly saves shelved dirstate just before aborting
current transaction, and restore dirstate with it after aborting by
utility function '_aborttransaction()' added by previous patch.

hgext/shelve.py

@@ -734,6 +734,8 b' def unshelve(ui, repo, *shelved, **opts)'
         repo.unfiltered().changelog.strip(oldtiprev, tr)
 
         unshelvecleanup(ui, repo, basename, opts)
+
+        _aborttransaction(repo)
     finally:
         ui.quiet = oldquiet
         if tr: