builtin/merge: avoid -Wformat-extra-args from ancient Xcode

d540b70c85 (merge: cleanup messages like commit, 2019-04-17) adds
a way to change part of the helper text using a single call to
strbuf_add_commented_addf but with two formats with varying number
of parameters.

this trigger a warning in old versions of Xcode (ex 8.0), so use
instead two independent calls with a matching number of parameters

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -830,9 +830,11 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 			strbuf_commented_addf(&msg, "\n");
 		}
 		strbuf_commented_addf(&msg, _(merge_editor_comment));
-		strbuf_commented_addf(&msg, _(cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS ?
-			scissors_editor_comment :
-			no_scissors_editor_comment), comment_line_char);
+		if (cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS)
+			strbuf_commented_addf(&msg, _(scissors_editor_comment));
+		else
+			strbuf_commented_addf(&msg,
+				_(no_scissors_editor_comment), comment_line_char);
 	}
 	if (signoff)
 		append_signoff(&msg, ignore_non_trailer(msg.buf, msg.len), 0);