shelve: restore shelved dirstate explicitly after aborting transaction

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

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.

This patch replaces 'if tr: tr.abort()' by 'lockmod.release(tr)',
because the former is already done in '_aborttransaction()' (and the
latter has no effect), if current transaction is aborted in it
successfully. Otherwise, the latter is enough to trigger aborting.

hgext/shelve.py

@@ -340,10 +340,10 b' def createcmd(ui, repo, pats, opts):'
             desc = util.ellipsis(desc, ui.termwidth())
         ui.status(_('shelved as %s\n') % name)
         hg.update(repo, parent.node())
+
+        _aborttransaction(repo)
     finally:
-        if tr:
-            tr.abort()
-        lockmod.release(lock, wlock)
+        lockmod.release(tr, lock, wlock)
 
 def cleanupcmd(ui, repo):
     """subcommand that deletes all shelves"""