We currently mark up auto-linked external links (source http://example.com/) as mw:ExtLink/URL (see http://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#External_links for the spec). There is however no need to distinguish between plain external and auto-linked external URLs on the client side if we automatically convert between the two forms as needed. So we should replace all uses of mw:ExtLink/URL with a plain mw:ExtLink and add some extra info to data-parsoid and cleverness to our serializer.
- If the target and the link text agree, we can serialize a modified or new link as an URL link in wikitext.
- If a former URL link's content text is modified by the editor and does not match the target any more, convert to a regular external link.
- If an unmodified external link *could* be rendered as an URL link, but is marked up as a full external link in the original Wikitext, then preserve this non-minimal serialization on round-trip.
Change detection requires custom shadowing similar to the logic we use for WikiLinks.