tag: Avoid autocreating multiple tag pages that vary only in capitalization. The...

Arguably, the real bug is in the interface to add_autofile, but since
that does take a filename, not a page name, it cannot really do case
handling on its own. The only other users of add_autofile in ikiwiki proper
is autoindex, and it always uses one case. Other third party plugins might
also need to add similar workarounds though.