bookmarks: factor method _printer out of for loop in printbookmarks This allows even further customization via extensions for printing bookmarks. For example, in hg-git this would allow printing remote refs by just modifying the 'bmarks' parameter instead of reimplementing the old commands.bookmarks method. Furthermore, there is another benefit: now multiple extensions can chain their custom data to bookmark printing. Previously, an extension could have conflicting bookmark output due to which loaded first and overrode commands.bookmarks. Now they can all play nicely together.