Make sure we always treat UPGRADING-* files as local links.

Because rt-static-docs is a depth-first search, it can find references
to UPGRADING-4.2 before it finds UPGRADING-4.2.  This results in a link
to metacpan.org rather than our upgrading doc.
Merge branch '4.2/pod-html-UPGRADING-links' into 4.2-trunk