Merge branch 'jc/show-usage-help'

The help text from "git $cmd -h" appear on the standard output for
some $cmd and the standard error for others.  The built-in commands
have been fixed to show them on the standard output consistently.

* jc/show-usage-help:
  builtin: send usage() help text to standard output
  oddballs: send usage() help text to standard output
  builtins: send usage_with_options() help text to standard output
  usage: add show_usage_if_asked()
  parse-options: add show_usage_with_options_if_asked()
  t0012: optionally check that "-h" output goes to stdout

builtin/credential.c

@@ -18,7 +18,8 @@ int cmd_credential(int argc,
 
 	git_config(git_default_config, NULL);
 
-	if (argc != 2 || !strcmp(argv[1], "-h"))
+	show_usage_if_asked(argc, argv, usage_msg);
+	if (argc != 2)
 		usage(usage_msg);
 	op = argv[1];