ci-windows: introduce manual windows CI
Note: most of the foundational work of this commit was done by Matt Harbison,
but enough has changed that I don't feel comfortable sending this patch as his.
In our quest to remove Python 2 from Mercurial, we need to get Windows CI going
to check that Python 3 support is up to an acceptable standard.
This is the first step: adding a manual step to run a full Windows CI, with a
certain definition of "full" since some things are not installed yet, like SVN.
Differential Revision: https://phab.mercurial-scm.org/D11025
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}