util: make sortdict.keys() return a copy stable

util: make sortdict.keys() return a copy

dict.keys() is documented to return a copy, so it's surprising that
sortdict.keys() did not. I noticed this because we have an extension
that calls readlocaltags(). That method tries to remove any tags that
point to non-existent revisions (most likely stripped). However, since
it's unintentionally working on the instance it's modifying, it
sometimes fails to remove tags when there are multiple bad tags in a
row. This was not caught because localrepo.tags() does an additional
layer of filtering.

sortdict is also used in other places, but I have not checked whether
its keys() and/or __delitem__() methods are used there.

--- a/mercurial/util.py	Mon Jan 30 22:50:20 2017 +0900
+++ b/mercurial/util.py	Mon Jan 30 22:58:56 2017 -0800
@@ -550,7 +550,7 @@
         except ValueError:
             pass
     def keys(self):
-        return self._list
+        return self._list[:]
     def iterkeys(self):
         return self._list.__iter__()
     def iteritems(self):