SCRIPTS: backport: use short revs and resolve the initial commit

SCRIPTS: backport: use short revs and resolve the initial commit

I find myself often getting trapped into calling "backport 2.0 HEAD" which
doesn't work because "HEAD" is passed as the argument to cherry-pick in
other repos. Let's resolve it first. And also let's shorten the commit IDs
to make the error messages more readable and to ease copy-paste.