By "remove a commit" do you mean rebase A to point at its previous commit, HEAD~1?
A commit SHA represents the entire DAG up until that point, not a given changeset. So in your example, branch B is still a pointer to a commit, and branch A is a pointer to a different commit. Where is the convenient lie?
A commit SHA represents the entire DAG up until that point, not a given changeset. So in your example, branch B is still a pointer to a commit, and branch A is a pointer to a different commit. Where is the convenient lie?