hg_delta_editor: fix symlink prefix confusion - SubversionRepo.get_file() strips the symlink prefix - Enforce that hg_delta_editor symlink data always contains the prefix. The alternative was seducing and more consistent with hg content but it makes the code more complicated since svn:special can be set before or after the content is set, and we need it in apply_textdelta() This issue fixes jQuery repository conversion at r3674.