xml: fulldiff target: best effort per-hunk RNG context

For instance, last hunk of diff -u resources-1.{0,2}.rng:

@@ -166,6 +213,7 @@ context: /<element-operations>/operations/op/@on-fail

<value>restart</value>
<value>standby</value>
<value>fence</value>

+ <value>restart-container</value>

    </choice>
  </attribute>
</optional>