git push %topic= allows non-owners to change topic

... while this is forbidden in the UI.

There are pros and cons for both allowing and disallowing non-owners to change these, but until recently, when Ahmad mangled by my qsharedpointer topic in an otherwise Q_FOREACH topic branch, the fact that I couldn't change the topic of someone else's commit was quite annoying.

So, I'd even say that git push shouldn't be allowed to change topics on existing changes (only when [NEW]), but that non-owners should be able to change the topic in the UI.

But as it is now, it's counter-intuitive and error-prone.