SCRIPTS: git-show-backports: add a restart-from-last option

SCRIPTS: git-show-backports: add a restart-from-last option

It's always a bit tricky to avoid already backported patches when they
just got a different ID (e.g. a critical fix in a topic branch). Most
often with stable topic branches we just want to pick all stable commits
since the last backported one. New option -L instead of -m does exactly
this: it enumerates only commits that were added to the reference branch
after its most recent backport.