t: introduce compatibility options to clar-based tests

Our unit tests that don't yet use the clar unit testing framework ignore
any option that they do not understand. It is thus fine to just pass
test options we set up globally to those unit tests as they are simply
ignored. This makes our life easier because we don't have to special
case those options with Meson, where test options are set up globally
via `meson test --test-args=`.

But our clar-based unit testing framework is way stricter here and will
fail in case it is passed an unknown option. Stub out these options with
no-ops to make our life a bit easier.

Note that this also requires us to remove the `-x` short option for
`--exclude`. This is because `-x` has another meaning in our integration
tests, as it enables shell tracing. I doubt there are a lot of people
out there using it as we only got a small hand full of clar tests in the
first place. So better change it now so that we can in the long run
improve compatibility between the two different test drivers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -353,6 +353,18 @@ struct option {
 	.callback = parse_opt_noop_cb, \
 }
 
+static char *parse_options_noop_ignored_value MAYBE_UNUSED;
+#define OPT_NOOP_ARG(s, l) { \
+	.type = OPTION_CALLBACK, \
+	.short_name = (s), \
+	.long_name = (l), \
+	.value = &parse_options_noop_ignored_value, \
+	.argh = "ignored", \
+	.help = N_("no-op (backward compatibility)"), \
+	.flags = PARSE_OPT_HIDDEN, \
+	.callback = parse_opt_noop_cb, \
+}
+
 #define OPT_ALIAS(s, l, source_long_name) { \
 	.type = OPTION_ALIAS, \
 	.short_name = (s), \