.arcconfig

extdata: add extdatasource reader

This adds basic support for extdata, a way to add external data
sources for revsets and templates. An extdata data source is simply a
list of lines of the form:

<revision identifier>[<space><freeform text>]\n

An extdata source is configured thusly:

[extdata]
name = <a url or path>

urls of the form shell: are launch shell commands to generate data.

This patch is slightly modified by Yuya Nishihara as follows:

 - fix typo
 - remove unused function
 - remove future expansion point for parameter (which can be added later
   as the extdata revset/template are experimental)

You can see the original patch at
https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-September/088426.html

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}