It should be possible to use anonymous git push to post comments
(created, say, by a ikiwiki-comment program). Currently, that is not
allowed, because users cannot edit, or create internal page files.
But, comments in allowed locations are an exception to that rule, and 
that exception should be communicated somehow to `IkiWiki::Receive`.
--[[Joey]] 

> Complications include: 
> 
> * Hard to see a way to prevent users from committing a comment that
>   claims to be written by someone else.
> * `checkcontent` hooks need to be run, but can't accept a comment
>   for later moderation, since it's coming in as part of a commit.
>   Best they could do is reject the commit.
