editor: handle property changes to links. Previously, property changes to links caused 'link ' to be prepended to the link destination. Removing a line that prepended it in Revision::set() appears to fix it. In these cases, the "file marked as link, but contains data" warning might be triggered. This should be safe, so it's lowered to a note and the language made less conclusive. In order to test this, extra revisions are added to the 'symlinks.svndump' fixture. As one of the new revisions add a link that points to 'link to this', a check that asserted that link destinations must not start with 'link ' was removed. This change is safe, as the test later on asserts exact equality with the contents of the 'links' dictionary.