MediaWiki.php: Factor out tryNormaliseRedirect
This is in preparation for fixing T67402, which requires adding
logic inside this condition block. However the to-be-added code
will influences whether or not a redirect should be made.
In case a redirect is not made, it has to fall through to the next
'elseif' handler in MediaWiki::performRequest(), which is not possible
from inside the 'if' block.
Hence, move it out in a separate block and use a boolean return value
to communicate whether the case has been handled.
This also allows us to unit test this thing. Which is desperately
needed. Albeit ugly as it requires lots of mocking.