Merge branch 'ds/format-patch-rfc-and-k' into maint-2.45

The "-k" and "--rfc" options of "format-patch" will now error out
when used together, as one tells us not to add anything to the
title of the commit, and the other one tells us to add "RFC" in
addition to "PATCH".

* ds/format-patch-rfc-and-k:
  format-patch: ensure that --rfc and -k are mutually exclusive

builtin/log.c

@@ -2050,8 +2050,10 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (cover_from_description_arg)
 		cover_from_description_mode = parse_cover_from_description(cover_from_description_arg);
 
-	if (rfc)
+	if (rfc) {
 		strbuf_insertstr(&sprefix, 0, "RFC ");
+		subject_prefix = 1;
+	}
 
 	if (reroll_count) {
 		strbuf_addf(&sprefix, " v%s", reroll_count);

t/t4014-format-patch.sh

@@ -1397,6 +1397,27 @@ test_expect_success '--rfc is argument order independent' '
 	test_cmp expect actual
 '
 
+test_expect_success '--subject-prefix="<non-empty>" and -k cannot be used together' '
+	echo "fatal: options '\''--subject-prefix/--rfc'\'' and '\''-k'\'' cannot be used together" >expect.err &&
+	test_must_fail git format-patch -1 --stdout --subject-prefix="MYPREFIX" -k >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect.err actual.err
+'
+
+test_expect_success '--subject-prefix="" and -k cannot be used together' '
+	echo "fatal: options '\''--subject-prefix/--rfc'\'' and '\''-k'\'' cannot be used together" >expect.err &&
+	test_must_fail git format-patch -1 --stdout --subject-prefix="" -k >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect.err actual.err
+'
+
+test_expect_success '--rfc and -k cannot be used together' '
+	echo "fatal: options '\''--subject-prefix/--rfc'\'' and '\''-k'\'' cannot be used together" >expect.err &&
+	test_must_fail git format-patch -1 --stdout --rfc -k >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect.err actual.err
+'
+
 test_expect_success '--from=ident notices bogus ident' '
 	test_must_fail git format-patch -1 --stdout --from=foo >patch
 '