parse-options: introduce OPT_HIDDEN_GROUP

Hidden options are not shown by `git <cmd> -h`, but are still shown by
`git <cmd> --help-all`. If there are a lot of hidden options or if they
don't belong to the same categories as other options, there is
currently no way to properly group them.

Using `OPT_GROUP("Foo")` means that "Foo" will always be shown which we
don't want if that group contains only hidden options.

To provide a way to have groups shown only when hidden options are
shown, let's implement an OPT_HIDDEN_GROUP macro.

To test this new macro, let's also improve `test-tool parse-options`
and test its output with `--help-all`.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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