Note: behavioral diffs between old BranchCommitMessage and new BranchCommitMessages

Leaving these as a memo for the record. None are likely to cause issues in practice, but they are real semantic differences from the previous code path.

1. Multi-line subject handling

• Old: git log -1 --format=%s <branch> returns the first line only.
• New: git for-each-ref --format=%(contents:subject) collapses newlines within the subject paragraph into spaces.
• Identical output for the standard "subject + blank line + body" convention. Diverges only for commits whose subject itself spans multiple lines without a blank line. The added test uses subject + blank + body, so this edge case is not covered.

2. Trailing whitespace

• Old: strings.TrimSpace(out) strips leading/trailing whitespace.
• New: only strings.TrimRight(out, "\n") runs before strings.Cut, so a trailing space in the subject would be preserved.
• Negligible in practice.

3. Error-fallback granularity

• Old: a per-branch git log failure only drops that branch's description.
• New: a single BranchCommitMessages error leaves commitMessages nil, so all branches fall back to no-message descriptions. for-each-ref with explicit patterns is unlikely to fail, but the granularity changed.

4. Short-name collision between refs/heads and refs/remotes (pathological)

• Old: git log -1 origin/foo follows git's DWIM rules (refs/heads wins), returning the local branch's subject.
• New: both refs hit the same %(refname:short) map key, and for-each-ref emits refs/heads/... before refs/remotes/..., so the remote-tracking entry overwrites and returns the remote subject instead.
• Only matters if a local branch is named like a remote-tracking ref (e.g. a local branch literally called origin/foo).

The completeBranches side using only refs/heads is fine on closer inspection. git worktree list --porcelain emits branch refs/heads/... for non-detached worktrees, so wt.Branch is structurally always a local branch short-name, and the refs/heads-only scope matches prior behavior exactly.