I think the tags svn and subversion should be synonyms.
Not just merged, because authors might search for "svn" and not "subversion", and vice versa.
The primary one should be subversion, because it seems to be the canonical name, and it’s what Wikipedia uses as article name. (Well, Wikipedia uses "Apache Subversion", so we could also use apache-subversion instead, similar to how we have microsoft-word, but I have no opinion on this.)