Add an 'empty' instruction

This is used to insert an empty commit into an integration branch.  For
example, in git.git the maintainer inserts an empty "### match next"
commit into the 'pu' branch to indicate that that point should be
tree-identical with the head of the 'next' branch.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: John Keeping <john@keeping.me.uk>