Don't move twice when moving subpages to a subpage

Don't move twice when moving subpages to a subpage

(bug 14817) When a page got moved to a subpage of itself, and "move
subpages" was checked, the page got moved, then moved again, like Foo ->
Foo/Bar -> Foo/Bar/Bar.  There was an explicit check to prevent this,
but the check was incorrect: it used $ot->getArticleID() after $ot had
already been moved, so the ID was the redirect.  The behavior of
getArticleID() here isn't obvious, so I cached the ID in advance for
clarity instead of switching to $nt->getArticleID().

Brief inspection of Title::moveSubpages() suggests that that would be
affected too.  The third place we have this code copy-pasted (bleh) is
SpecialRenameuser, but that's not affected because usernames can't have
slashes in them.