Record merge progress durably and finish pushed merges at startup

internal/git/notes.go

@@ -69,7 +69,23 @@ func NoteCommit(repo, commit, note string, env []string) (string, error) {
 	return runEnv(repo, env, append(commitArgs, tree)...)
 }
 
-func SetNotes(repo, notes string) error {
-	_, err := run(repo, "update-ref", NotesRef, notes)
+func AdvanceNotes(repo, notes string) error {
+	current, stderr, code, err := runCode(repo, nil, "rev-parse", "--verify", "--quiet", NotesRef)
+	if err != nil {
+		return err
+	}
+	if code != 0 && code != 1 {
+		return fmt.Errorf("git rev-parse %s: exit status %d: %s", NotesRef, code, stderr)
+	}
+	if code == 0 {
+		contained, err := IsAncestor(repo, notes, current)
+		if err != nil {
+			return err
+		}
+		if contained {
+			return nil
+		}
+	}
+	_, err = run(repo, "update-ref", NotesRef, notes)
 	return err
 }