parseopt: add OPT_NOOP_NOARG

Add OPT_NOOP_NOARG, a helper macro to define deprecated options in a
standard way.  The help text is taken from the no-op option -r of
git revert.

The callback could be made to emit a (conditional?) warning later.  And
we could also add OPT_NOOP (requiring an argument) etc. as needed.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options-cb.c

@@ -123,3 +123,8 @@ int parse_opt_string_list(const struct option *opt, const char *arg, int unset)
 	string_list_append(v, xstrdup(arg));
 	return 0;
 }
+
+int parse_opt_noop_cb(const struct option *opt, const char *arg, int unset)
+{
+	return 0;
+}