MergeOp: always use allowClosed=false when bypassing submit rules

Submit rules can be bypassed for a change by doing a "git push" with the
%submit option. The implementation in MergeOp calls submit rules with
the allowClosed option; for the first trial we pass allowClosed=false.
For retries we pass allowClosed=true.

In this change, we update this method and skip checking for submit rules
if the change is closed. I'm not exactly sure why the code was
implemented this way, but I assume that this was needed when we try to
submit a changeset (i.e. many changes), so after a retry, some changes
might have been merged while others haven't been. When we do a retry, we
can just skip those changes that are closed since their submit records
are stored in NoteDb anyway. For new changes, we always use
allowClosed=false when requesting submit records.

Change-Id: I77b753caac6f670934bdca54387babff9c189e29