magit-diff-highlight-recursive: cosmetics
magit-diff-wash-revision: bugfix
magit-revision-insert-related-refs: new function
magit-get-{current,next}-tag: add optional REV argument
magit-revision-insert-related-refs: as a pretty group

Use a new section as the parent of all sections representing related
refs.  Make them prettier, taking inspiration from how the diffstat
is displayed.  Move the new section just before the diffstat.