clean up use of IkiWiki::Receive

clean up use of IkiWiki::Receive

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.