Show usage string for 'git grep -h'

Clarification: the following description only talks about "git
grep -h" without any other options and arguments.

Such a change cannot be breaking backward compatibility.  "grep
-h" cannot be asking for suppressing filenames, as there is no
match pattern specified.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -788,6 +788,13 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	/*
+	 * 'git grep -h', unlike 'git grep -h <pattern>', is a request
+	 * to show usage information and exit.
+	 */
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(grep_usage, options);
+
 	memset(&opt, 0, sizeof(opt));
 	opt.prefix = prefix;
 	opt.prefix_length = (prefix && *prefix) ? strlen(prefix) : 0;