Nothing. Conveniently, git allows you to do exactly that. 1. Checkout B. 2. Merge A into B. 3. Git tells you there's a merge conflict and what to do to fix it:

Nothing. Conveniently, git allows you to do exactly that. 1. Checkout B. 2. Merge A into B. 3. Git tells you there's a merge conflict and what to do to fix it: "Auto-merging foo CONFLICT (content): Merge conflict in foo Automatic merge failed; fix conflicts and then commit the result." 4. Edit foo and fix the conflict. 5. "git add foo" 6. Commit. What's the problem?