Theory

Theory: A Branch is a Name; a Merge is a Node with Two Parents

Creating a branch does not copy files. It writes a ref. Merging either slides that ref (fast-forward) or writes a commit with two parents (a join).

Invariants

1. git switch -c feat = write refs/heads/feat at current HEAD, point HEAD at it.
2. Fast-forward is legal iff the current tip is an ancestor of the other tip.
3. A three-way merge diffs base→ours and base→theirs and applies both to the base. The merge base is the unique best common ancestor (usually).
4. Conflict markers are unmerged index stages (1 base, 2 ours, 3 theirs) spilled into the working tree.
5. During a merge, --ours is HEAD. During a rebase, --ours is the upstream. That swap is the theory, not a bug.

 fast-forward          three-way merge

 A ← B ← C   main      A ← B ← C ← M   main
           ← D ← E           ↑     ↑
                             D ← E

Why merge commits exist

They record that two lines of work met, and they keep both sides’ hashes stable. Rebase prefers a straight first-parent chain and pays with new hashes. Neither changes the fact that Git stores snapshots.

What can break

• Merging with uncommitted work that overlaps the incoming tree.
• Resolving markers but forgetting git add (merge stays open).
• git merge --abort after the merge commit exists (too late).

How to read this chapter

Create/switch, merge types, three-way, conflicts, fast-forward, lab; then mergetool, ours/theirs, orphan, comparing branches, rerere.