RedirectSpecialPage: handle interwiki redirects.

RedirectSpecialPage: handle interwiki redirects.

Previously, WikiPage::performRequest() would assume that Titles returned
by RedirectSpecialPage::getRedirect() are local pages, and would set
$wgTitle to whatever was returned. That would lead to a confused state
where the skin would try to render for an interwiki Title.

Instead, WikiPage::performRequest() should wrap the interwiki redirect
in a call to Special:GoToInterwiki/xyz, just like
Title::getFullUrlForRedirect() does, but still avoid the HTTP redirect,
to avoid leaking private information via view counters (T109724).

There are two things to test:
1) call Special:MyLanguage with an interwiki prefix,
   e.g. Special:MyLanguage/wikipedia:XYZ.
2) create a page that contains an interwiki redirect,
   e.g. #REDIRECT [[wikipedia:XYZ]], then call Special:MyLanguage
   for that page.

For these tests, the user language should be the same as the content
language. That is the critical case. If the user language differs
from the content language, the problem would be obscured by another
bug which is addressed by Ib4cbeec47a877c473.

Bug: T227700
Change-Id: I2852c5a9774f0c76e49f1e3876fcfe85a305f9ce