Avoid the generic "you are not allowed to change" message,
and instead allow check_canedit to propigate out useful error messages.
Went back to calling check_canedit in fatal mode, but added a parameter to
avoid calling the troublesome subs that might cause a login attempt.
Everywhere that REMOTE_ADDR was used, a session object is available, so
instead use its remote_addr method.
In IkiWiki::Receive, stop setting a dummy REMOTE_ADDR.
Note that it's possible for a session cookie to be obtained using one IP
address, and then used from another IP. In this case, the first IP will now
be used. I think that should be ok.
Many calls to file_prune were incorrectly calling it with 2 parameters.
In cases where the filename being checked is relative to the srcdir,
that is not needed.
Made absolute filenames be pruned. (This won't work for the 2 parameter call
style.)
Loading and use of IkiWiki::Receive can all be pushed into the git plugin,
rather than scattered around.
I had at first wanted to make a receive plugin and move it there,
but a plugin was not a good fit; you don't want users to have to manually
load it, and making the git plugin load the receive plugin at the right
times would need more, and ugly code.