SCRIPTS: add a new "backport" script to simplify long series of backports

SCRIPTS: add a new "backport" script to simplify long series of backports

The script is simply called from the repository holding the patch to
backport, with the last branch number and the commit(s) ID(s) to send
there and it then follows the chain of "down" repos to go down one step
until it meets the indicated last one. It basically automates what we do
by hand. Example:

   ./scripts/backport 1.9 1c7c0d6b97513e79c304aaf834f83843f32a674d

Note that it does *not* push, which still has to be done by hand after
building and testing.