changing-files: add a utility to compute the merged files post-commit
We will need it in `_getsidedata` as soon as we start persisting that set.
Differential Revision: https://phab.mercurial-scm.org/D9089
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}