[rfc] help on formats should indicate default

[John Arbash Meinel]

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I think this is a general statement about all of our new RegistryOption
options. When you do:

bzr help log

It gives you the list of possible options, but it doesn't tell you that
'--long' is the default.

I realize this is probably tricky, since it seems the default is
actually set per-branch, and not anywhere sensible like on the registry
itself.

James Troup ran into it, and posted this bug:

https://launchpad.net/bugs/84763

I saw a similar thing with --merge-type, since the default is set by:

        if merge_type is None:
            merge_type = _mod_merge.Merge3Merger

In cmd_merge.run().

Would it be reasonable to at least tell the Registry what the default
is, and then the help formatter can use that as a hint?

Do we actually need --log-format to have a per-branch default? I can see
maybe having a global default.

Then again, I'd be okay with just having default be the normal default,
and if someone has gone to the effort of configuring a different log
formatter for their branch, I'll let them figure out why default isn't
the standard default.

John
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