Show usage string for 'git check-ref-format -h'

This only changes the behavior of "git check-ref-format -h"
without any other options and arguments.

This change cannot be breaking backward compatibility, since any
valid refname must contain a /.   Most existing scripts use
arguments such as "heads/$foo".  If some script checks the
refname "-h" alone, git check-ref-format will still exit with
nonzero status, and the only detrimental side-effect will be a
usage string sent to stderr.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-check-ref-format.c

@@ -31,6 +31,9 @@ static void collapse_slashes(char *dst, const char *src)
 
 int cmd_check_ref_format(int argc, const char **argv, const char *prefix)
 {
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage(builtin_check_ref_format_usage);
+
 	if (argc == 3 && !strcmp(argv[1], "--branch")) {
 		struct strbuf sb = STRBUF_INIT;