Week 4 - M3: Branch checkout + fast-forward merge

[aman-a-shah]

Owner: @DanisLol
Files: minigit/commits.py, tests/test_commits.py
Depends on: #18 and #23 (M2 checkout)
Branch: week4/m3-checkout-and-fast-forward

Make branch switching restore files. Add fast-forward merging: move the current branch to a later commit without creating a merge commit.

Steps

1. Start from updated main after Week 3 - M3: Real tree commits + history helpers #18 merges. Use a fake checkout for ordering tests until M2 lands.
2. Before a switch or fast-forward, compare the index with get_head_tree().
   • Treat an unborn HEAD as an empty tree.
   • Staged changes raise MiniGitError.
   • Let M2 reject unstaged changes and untracked files that would be overwritten.
3. Update switch_branch(name): read the target ref, read its commit, call self.tree.checkout(commit.tree), then write HEAD.
   • Missing branch: RefNotFoundError.
   • Failed checkout: HEAD and refs stay unchanged.
4. Add is_ancestor(ancestor_hash: str, descendant_hash: str) -> bool.
   • Reuse walk_history; follow every parent.
   • A commit is its own ancestor. Share this helper with M4.
5. Implement merge(branch_name) in this order:
   • Missing target: RefNotFoundError.
   • Target equals the current tip, or is already its ancestor: return None; change nothing.
   • Current tip is the target's ancestor, or the current branch is unborn: checkout the target tree, then update the current branch ref. Return None.
   • Otherwise raise MiniGitError("three-way merge is not implemented yet"); change nothing.
6. Register minigit merge <branch>. Keep HEAD on the current branch after merging. Branch creation and listing keep their existing behavior.
7. Run scripts/quality-check.sh. Open a PR linked to this issue and request a teammate review.

Tests

• Switch between two real snapshots; check files, index, HEAD, and unchanged branch tips.
• Fast-forward updates the current ref and files without creating a commit.
• Check same-tip merges, already-merged targets, unborn branches, and diverged history.
• Check ancestry through a second parent.
• Staged edits or checkout failure leave refs and HEAD unchanged.

Team checkpoint

Commit A on main, create feature, switch to it, and commit B. Switch to main: files show A. Merge feature: files show B, both refs point to B, and status is clean.

Done when

• Branch switching and fast-forward merge pass with real M1/M2.
• Ancestry helper is ready for M4.
• Team checkpoint passes.
• Quality check passes; PR open and review requested.