Local Parent POM not found if <relativePath> specifies a directory

The Maven core allows to specify a directory for the <relativePath> element in a module POM to locate the parent POM, e.g.

<parent>
    ...
    <relativePath>../parent</relativePath>
</parent>

will properly find the parent POM in "../parent/pom.xml". However, the Site plugin does not follow this lookup strategy:

[INFO] [site:site]
[INFO] Unable to load parent project from a relative path: Could not find the model file '[SNIP]\..\parent'. for project unknown
[INFO] Parent project loaded from repository.

This log output is actually from 2.0-beta-6-SNAPSHOT, 2.0-beta-5 outputs a different message but fails, too.

The attached patch fixes this although I wonder whether this functionality is not already included somewhere in the Maven core (where is belongs IMHO).