Path expressions on match templates don't work with path segments encountered before the match directive

[cmlenz]

Before [18], the following worked:

<div xmlns:py="http://purl.org/kid/ns#">
  <span py:match="div/greeting">
    Hello ${select('@name')}
  </span>
  <greeting name="Dude" />
</div>

Since [18], the patch expression “div/greeting” no longer matches because the <div> element is encountered before the py:match directive in the input. This should probably be fixed, though. One option is to keep a stack of already encountered elements in the Template._match filter and push them into the path evaluator so that it has the correct context.