C++: prevent possibly highlighting a document twice

Possible when the highlighter does not need semantic info to run, and is
started through onDocumentUpdated() and updateDocumentNow().

Change-Id: I720299730213ac196143a273fb60cee8e43111f1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cpptoolseditorsupport.cpp

@@ -392,10 +392,14 @@ void CppEditorSupport::startHighlighting()
         m_lastHighlightRevision = revision;
         emit highlighterStarted(&m_highlighter, m_lastHighlightRevision);
     } else {
+        const unsigned revision = currentSource(false).revision;
+        if (m_lastHighlightRevision == revision)
+            return;
+
+        m_lastHighlightRevision = revision;
         static const Document::Ptr dummyDoc;
         static const Snapshot dummySnapshot;
         m_highlighter = m_highlightingSupport->highlightingFuture(dummyDoc, dummySnapshot);
-        m_lastHighlightRevision = editorRevision();
         emit highlighterStarted(&m_highlighter, m_lastHighlightRevision);
     }
 }