test: avoid 'test-check-commit.t' complaining copied code We switch to git diff.

topic: small test cleanuo The section was repeated.

topic: move the heads data to the branch line when appropriates The topic line is dropped as it is quite confusing.

topic: add some initial support for using stack on named branch Stack is a useful command that can make sense in other context. The current support is hacky and awful, but we have something!