Someone tried to report a bug using IRC while I was on vacation.
--[[Joey]] 

<pre>
julm: [11:58:35] han, it's me the problem; I was generating a post-update hook instead of a pre-receive hook
julm: [12:03:59] why does the pre-receive hook return: "Status: 302 Found" and a "Location: <url>"? Is it normal?
julm: [00:08:44] it's Plugin/httpauth.pm which is outputing those Status and Location
julm: [00:09:12] problem is that it's an anonymous push via git://
julm: [03:28:53] hacked my way to fix it somehow: http://git.internet.alpes.fr.eu.org/?p=web/ikiwiki.git;a=commitdiff;h=7211df4f7457c3afab53822a97cbd21825c473f4
</pre>

Analysis: 

* IkiWiki::Receive calls `check_canedit`.
* httpauth's canedit hook returns an error handler function
  which redirects the browser through the cgiauthurl.
  (Similarly, signinedit's hook may call needsignin, which
  can display a signin form.
* That doesn't work well when doing a git anon push. :)
* Also, IkiWiki::Receive calls `check_canattach` and
  `check_canremove`, which both also call `check_canedit`.

So, all these calls need to avoid running the error handler
functions returned by canedit hooks, and just return error
messages.