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