xmerge: reuse the value of node.getNodeName() and remove duplicated constants

Change-Id: Ia835139496ab8e5230a09df2caf23637a5559ba4
Reviewed-on: https://gerrit.libreoffice.org/11836
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>