VCS/Diff editors: Set readonly attributes correctly.

Make VCS-generated editors read-only, enable editing
when opening a patch.
Make VCS-Editor non-read-only by default, add setter
for "Forced read-only" that makes it a temporary,
read-only file.

Task-number: QTCREATORBUG-1528
Reviewed-by: Thorbjorn Lindeijer <thorbjorn.lindeijer@nokia.com>

src/plugins/git/gitclient.cpp

@@ -195,6 +195,7 @@ VCSBase::VCSBaseEditor
             rc->setCodec(VCSBase::VCSBaseEditor::getCodec(source));
     }
     m_core->editorManager()->activateEditor(outputEditor);
+    rc->setForceReadOnly(true);
     return rc;
 }