i#84172 RTF import: handle nested groups inside redlines

The problem was that in case we had {\revised foo {bar}}, then we only
started a redline before "foo", but we ended it twice: during handling
of both "}". Make sure that the end of the redline is only handled by
the second one.

Change-Id: I41ab8b486e8615d3479fa0fadd6016476859b4b2

writerfilter/qa/cppunittests/rtftok/data/pass/i84172.rtf

+{\rtf1
+{\*\revtbl
+{Unknown;}
+}
+\par \pard\plain
+{
+{\revised\revauth1\revdttm-1497631607 foo
+{bar}
+}
+}
+}

writerfilter/source/rtftok/rtfdocumentimpl.cxx

@@ -4877,6 +4877,11 @@ int RTFDocumentImpl::pushState()
         break;
     }
 
+    // If this is true, then ooxml:endtrackchange will be generated.  Make sure
+    // we don't generate more ooxml:endtrackchange than ooxml:trackchange: new
+    // state does not inherit this flag.
+    m_aStates.top().bStartedTrackchange = false;
+
     return 0;
 }