Given that some OSes have bash in /usr/local/bin and in order not to
give too easy an excuse to Olivier for not backporting fixes, let's
make a few scripts rely on /usr/bin/env bash instead of /bin/bash :-)
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 1c7c0d6b97
Note that it does *not* push, which still has to be done by hand after
building and testing.
