dirstate: track that changes are pending in a transaction
Nothing is currently broken because if this, but this make the
`_invalidated_context` attribute more accurate.
Being more accurate here will help us later, when dealing with `status` call.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}