mkrockspecs: git magic to reenable slingshot submodule in PKG-git-1.rockspec.

Without this changeset, it was not possible to run bootstrap
inside a non-release revision checked out by luarocks install.
* mkrockspecs.in (prog): New --branch/-b option.
(build): Ugly git magic to fetch the missing slingshot submodule
from `luarocks install http://raw.github.com/owner/package.git`.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>