(bug 41990) Fix regression in API edit of redirect

(bug 41990) Fix regression in API edit of redirect

Before ContentHandler, an API edit passing a redirect for title and
redirect=true and omitting basetimestamp would use the last revision
timestamp of the target page. When ContentHandler was merged, this was
accidentally changed to use the last revision timestamp of the redirect
page, which was likely much earlier than the target page's last revision
and so causes an edit conflict in almost all cases.

Some scripts took advantage of this along with appendtext to add notices
to users' talk pages, so restore that old behavior. We'll also adjust
the contentmodel/contentformat detection added by ContentHandler to use
the model and format of the target page rather than the redirect in this
case, as that seems more likely to be less wrong.

Change-Id: If0c674e26a4deb54ec14f0bf45418d666a397347