Add Special:Diff as an internally-linkable redirect to diff pages

Add Special:Diff as an internally-linkable redirect to diff pages

This is similar to Special:PermanentLink added in r79036 and has been
asked for several times in different places, including:
- on the English Wikipedia (oldid=539308532)
- on mediawiki.org (lqt_oldid=31691)
- on the French Wikipedia (oldid=93029892)
- on the English Wikipedia again (oldid=588408888)

A notable use-case is linking to diffs in the edit summaries, where
external links are not yet allowed (bug 14892).

All of the following are valid usages:
- [[Special:Diff/12345]] (diff of a revision with the previous one)
- [[Special:Diff/12345/prev]] (diff of a revision with the previous one as well)
- [[Special:Diff/12345/next]] (diff of a revision with the next one)
- [[Special:Diff/12345/cur]] (diff of a revision with the latest one of that page)
- [[Special:Diff/12345/98765]] (diff between arbitrary two revisions)

Co-authored-by: Jérémie Roquet <jroquet@arkanosis.net>
Co-authored-by: Bartosz Dziewoński <matma.rex@gmail.com>
Change-Id: I77fdaf8e04375caa1d67ca4a3ec3bd93920c3309