[PATCH 1 of 2 V2] formatter: reserve _ prefixed keys as internal and don't render

[Yuya Nishihara]

On Wed, 22 Mar 2017 16:21:34 +0000, Ryan McElroy wrote:
> This seems like a good time to bring up by long-held desire for a JSON 
> template that allows specifying what keys you're interested in. This 
> would solve this problem more suitably than internal name-munging.
> 
> To expand, -T json is insufficient for some things, like requesting file 
> adds from hg log, which might be slow to ask for in large repos. I'd 
> like a syntax like:
> 
> -T json(desc,fileadds)

Suppose template keywords can be lazily evaluated, this will be doable by
templater.

1. add jsonobj(*keywords) function that evaluates each keyword and dump
   them as JSON object:

    jsonobj(desc, fileadds)
    => '{"desc": desc, "fileadds": fileadds}'

2. add :docheader, :docfooter, and :separator options

    [templates]
    json = "{jsonobj(desc, fileadds)}"
    json:docheader = "[\n"
    json:docfooter = "]\n"
    json:separator = ",\n"

3. add function-like template specifier

    [templates]
    json(*args) = "{jsonobj(*args)}"

4. make json(*args) a builtin template

Perhaps (3) isn't easy, but we can just hard-code parsing of "json(...)",
and implement the general form later.