Add "git: {tag: a_tag}" code inclusion parameter

This changes the syntax for including a git tagged file from
tag: a_tag
to
git: {tag: a_tag}.
Support for the old syntax has been removed so If anyone other than me
is using it, they’re in for a nasty surprise.  As this seems deeply
unlikely,I think it’s simplest to add the new syntax and remove the old
in this one commit.