sequencer: factor out code to append squash message

This code is going to grow over the next two commits so move it to
its own function.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -1718,6 +1718,17 @@ static int is_pick_or_similar(enum todo_command command)
 	}
 }
 
+static void append_squash_message(struct strbuf *buf, const char *body,
+				  struct replay_opts *opts)
+{
+	unlink(rebase_path_fixup_msg());
+	strbuf_addf(buf, "\n%c ", comment_line_char);
+	strbuf_addf(buf, _("This is the commit message #%d:"),
+		    ++opts->current_fixup_count + 1);
+	strbuf_addstr(buf, "\n\n");
+	strbuf_addstr(buf, body);
+}
+
 static int update_squash_messages(struct repository *r,
 				  enum todo_command command,
 				  struct commit *commit,
@@ -1779,12 +1790,7 @@ static int update_squash_messages(struct repository *r,
 	find_commit_subject(message, &body);
 
 	if (command == TODO_SQUASH) {
-		unlink(rebase_path_fixup_msg());
+		append_squash_message(&buf, body, opts);
-		strbuf_addf(&buf, "\n%c ", comment_line_char);
-		strbuf_addf(&buf, _("This is the commit message #%d:"),
-			    ++opts->current_fixup_count + 1);
-		strbuf_addstr(&buf, "\n\n");
-		strbuf_addstr(&buf, body);
 	} else if (command == TODO_FIXUP) {
 		strbuf_addf(&buf, "\n%c ", comment_line_char);
 		strbuf_addf(&buf, _("The commit message #%d will be skipped:"),