@@ -1892,6 +1892,32 @@ static const char skip_first_commit_msg_str[] = N_("The 1st commit message will
 static const char skip_nth_commit_msg_fmt[] = N_("The commit message #%d will be skipped:");
 static const char combined_commit_msg_fmt[] = N_("This is a combination of %d commits.");
 
+void add_squash_combination_header(struct strbuf *buf, int n)
+{
+	strbuf_addf(buf, "%s ", comment_line_str);
+	strbuf_addf(buf, _(combined_commit_msg_fmt), n);
+}
+
+void add_squash_message_header(struct strbuf *buf, int n, int skip)
+{
+	strbuf_addf(buf, "%s ", comment_line_str);
+	if (n == 1)
+		strbuf_addstr(buf, skip ? _(skip_first_commit_msg_str) :
+				   _(first_commit_msg_str));
+	else
+		strbuf_addf(buf, skip ? _(skip_nth_commit_msg_fmt) :
+			    _(nth_commit_msg_fmt), n);
+}
+
+size_t squash_subject_comment_len(const char *body, int squashing)
+{
+	if (starts_with(body, "amend!") ||
+	    (squashing && (starts_with(body, "squash!") ||
+			   starts_with(body, "fixup!"))))
+		return commit_subject_length(body);
+	return 0;
+}
+
 static int is_fixup_flag(enum todo_command command, unsigned flag)
 {
 	return command == TODO_FIXUP && ((flag & TODO_REPLACE_FIXUP_MSG) ||

@@ -2005,20 +2031,13 @@ static int append_squash_message(struct strbuf *buf, const char *body,
 {
 	struct replay_ctx *ctx = opts->ctx;
 	const char *fixup_msg;
-	size_t commented_len = 0, fixup_off;
-	/*
-	 * amend is non-interactive and not normally used with fixup!
-	 * or squash! commits, so only comment out those subjects when
-	 * squashing commit messages.
-	 */
-	if (starts_with(body, "amend!") ||
-	    ((command == TODO_SQUASH || seen_squash(ctx)) &&
-	     (starts_with(body, "squash!") || starts_with(body, "fixup!"))))
-		commented_len = commit_subject_length(body);
+	size_t commented_len, fixup_off;
+
+	commented_len = squash_subject_comment_len(body,
+				command == TODO_SQUASH || seen_squash(ctx));
 
-	strbuf_addf(buf, "\n%s ", comment_line_str);
-	strbuf_addf(buf, _(nth_commit_msg_fmt),
-		    ++ctx->current_fixup_count + 1);
+	strbuf_addch(buf, '\n');
+	add_squash_message_header(buf, ++ctx->current_fixup_count + 1, 0);
 	strbuf_addstr(buf, "\n\n");
 	strbuf_add_commented_lines(buf, body, commented_len, comment_line_str);
 	/* buf->buf may be reallocated so store an offset into the buffer */

@@ -2083,9 +2102,8 @@ static int update_squash_messages(struct repository *r,
 		eol = !starts_with(buf.buf, comment_line_str) ?
 			buf.buf : strchrnul(buf.buf, '\n');
 
-		strbuf_addf(&header, "%s ", comment_line_str);
-		strbuf_addf(&header, _(combined_commit_msg_fmt),
-			    ctx->current_fixup_count + 2);
+		add_squash_combination_header(&header,
+					      ctx->current_fixup_count + 2);
 		strbuf_splice(&buf, 0, eol - buf.buf, header.buf, header.len);
 		strbuf_release(&header);
 		if (is_fixup_flag(command, flag) && !seen_squash(ctx))

@@ -2109,12 +2127,9 @@ static int update_squash_messages(struct repository *r,
 			repo_unuse_commit_buffer(r, head_commit, head_message);
 			return error(_("cannot write '%s'"), rebase_path_fixup_msg());
 		}
-		strbuf_addf(&buf, "%s ", comment_line_str);
-		strbuf_addf(&buf, _(combined_commit_msg_fmt), 2);
-		strbuf_addf(&buf, "\n%s ", comment_line_str);
-		strbuf_addstr(&buf, is_fixup_flag(command, flag) ?
-			      _(skip_first_commit_msg_str) :
-			      _(first_commit_msg_str));
+		add_squash_combination_header(&buf, 2);
+		strbuf_addch(&buf, '\n');
+		add_squash_message_header(&buf, 1, is_fixup_flag(command, flag));
 		strbuf_addstr(&buf, "\n\n");
 		if (is_fixup_flag(command, flag))
 			strbuf_add_commented_lines(&buf, body, strlen(body),

@@ -2133,9 +2148,8 @@ static int update_squash_messages(struct repository *r,
 	if (command == TODO_SQUASH || is_fixup_flag(command, flag)) {
 		res = append_squash_message(&buf, body, command, opts, flag);
 	} else if (command == TODO_FIXUP) {
-		strbuf_addf(&buf, "\n%s ", comment_line_str);
-		strbuf_addf(&buf, _(skip_nth_commit_msg_fmt),
-			    ++ctx->current_fixup_count + 1);
+		strbuf_addch(&buf, '\n');
+		add_squash_message_header(&buf, ++ctx->current_fixup_count + 1, 1);
 		strbuf_addstr(&buf, "\n\n");
 		strbuf_add_commented_lines(&buf, body, strlen(body),
 					   comment_line_str);

@@ -208,6 +208,29 @@ int todo_list_rearrange_squash(struct todo_list *todo_list);
  */
 void append_signoff(struct strbuf *msgbuf, size_t ignore_footer, unsigned flag);
 
+/*
+ * Append the "This is a combination of N commits." banner that "git rebase
+ * -i" writes at the top of a squashed commit's message, commented out with
+ * the comment character.
+ */
+void add_squash_combination_header(struct strbuf *buf, int n);
+
+/*
+ * Append the header (1-based N) that "git rebase -i" writes above each message
+ * when squashing, commented out with the comment character. With SKIP it reads
+ * "The ... commit message will be skipped" for a message that is dropped (a
+ * fixup), otherwise "This is the ... commit message".
+ */
+void add_squash_message_header(struct strbuf *buf, int n, int skip);
+
+/*
+ * Return the length of the leading subject of BODY when it should be commented
+ * out in a squash message, or 0 otherwise. An "amend!" subject always
+ * qualifies; "squash!" and "fixup!" subjects only when SQUASHING, since a
+ * plain fixup chain keeps them.
+ */
+size_t squash_subject_comment_len(const char *body, int squashing);
+
 void append_conflicts_hint(struct index_state *istate,
 		struct strbuf *msgbuf, enum commit_msg_cleanup_mode cleanup_mode);
 enum commit_msg_cleanup_mode get_cleanup_mode(const char *cleanup_arg,