[PATCH 1 of 2 STABLE] tests: demonstrate `hg show` mapping to showconfig

[Yuya Nishihara]

On Thu, 27 Apr 2017 16:11:34 -0700, Gregory Szorc wrote:
> This series is a "pick your poison" if you are a reviewer. I'm partial to
> the more complex one adding an argument to @command. But it may not be
> appropriate for stable. So a hacky fallback is provided as an alternative.

I don't think either of them is good for stable. show is highly experimental
extension, right?

That said, the safest hack I can think of is adding null show command to
the default command table, and wrap it by the show extension.