relnotes/13.0R: Use gitref macro instead of shortcodes

The gitref macro added in commit fb36cfdfb2 handles changes in both src,
doc and ports trees, takes up less space, and is quicker to write, so
update all uses of the commit shortcode.

While here, also update the canonical sample that's commented out.

Also re-arrange some items which were initially placed in the wrong
sections.

Since the shortcode is now no longer used, remove it.