adapt it to latest gitpython api; many fixes

@ben.boeckel those changes made it work in my machine.

The backport commits are correcltly generate, however, it leaves unstages changes in the workdir. I cannot find out why

IF i reset --hard I can push it and continue the backporting in gitlab

Signed-off-by: Vicente Adolfo Bolea Sanchez vicente.bolea@kitware.com