Improve "checkpatch-feature-branch.sh" to support accepting
range as argument.

The main purpose of "checkpatch-feature-branch.sh" is to test all
patches of a feature branch. When we run the script against master
(or nm-1-*), then there is no feature branch.

Previously, the script would just error out.

That is not very useful, in particular as we call this from gitlab-ci,
which also runs on master.

Instead, in that case, test the HEAD.

This takes current HEAD branch, and finds all the commits what
are not on master or one of the nm-1-* branches, and runs
checkpatch.pl on each.

The use is to run checkpatch.pl on all patches of a feature
branch.