tag: When tagpage is set, force the links created by tagging to point at the toplevel...

The html links already went there, but internally the links were not
recorded as absolute, which could cause confusing backlinks etc.

For example, with tagbase=tags, if blog/tags/bar existed and blog/foo was
tagged bar, it would link to /tags/bar. But, the link would be recorded
simply as a link to tags/bar, and so later blog/tags/bar would appear to
have the backlink.