SCRIPTS: git-show-backports: emit the shell command to backport a commit

SCRIPTS: git-show-backports: emit the shell command to backport a commit

It's cumbersome to copy-paste a commit ID into another window after having
typed "git cherry-pick -sx", so let's have the suggested output format of
git-show prepare this line just before the subject line, it remains at a
stable position on the terminal when searching for "/^commit". One just
has to copy-paste the line into another terminal will result in the commit
being properly picked.