Merges for DuckDB branches and snapshots

differential 1.1 merges branches cell by cell. Conflicts are resolved with a table, schemas can change, and results go back into main, locally or through S3. Since the first post about differential, the answer to “how do I get my branch back into main?” has been: you don’t, not really. differential_restore can adopt a branch’s entire state, but that throws away everything main did in the meantime. The 1.0 post said so plainly in its list of limits: No merge. Branches and conflicts are kept, never merged automatically. And the diff post stopped exactly where a merge would begin. You could see what a branch changed, but bringing those changes back was SQL you had to write yourself. Version 1.1 closes that gap with three-way merges: differential_merge_branch merges two branches. It finds their common base in the recorded history and writes the result to a new branch or back into the target branch, fast-forwarding where it can. differential_merge_snapshots merges three snapshots you pick…