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.