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
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true