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

Fix a bug that allows the "--rfc" and "-k" options to be specified together
when "git format-patch" is executed, which was introduced in the commit
e0d7db7 ("format-patch: --rfc honors what --subject-prefix sets").

Add a couple of additional tests to t4014, to cover additional cases of
the mutual exclusivity between different "git format-patch" options.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Dragan Simic

builtin/log.c

@@ -2044,8 +2044,10 @@ 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
 '