cmdutil: satisfy expections in dirstateguard.__del__, even if __init__ fails

Python "delstructors" are terrible - this one because it assumed that __init__
had completed before it was called. That would not necessarily be the case if
the repository was read only or broken and saving the dirstate thus failed in
unexpected ways. That could give confusing warnings about missing '_active'
after failures.

To fix that, make sure all member variables are "declared" before doing
anything that possibly could fail. [Famous last words.]

mercurial/cmdutil.py

@@ -3526,10 +3526,11 @@ class dirstateguard(object):
 
     def __init__(self, repo, name):
         self._repo = repo
+        self._active = False
+        self._closed = False
         self._suffix = '.backup.%s.%d' % (name, id(self))
         repo.dirstate.savebackup(repo.currenttransaction(), self._suffix)
         self._active = True
-        self._closed = False
 
     def __del__(self):
         if self._active: # still active