dirstate: make DirstateItem constructor accept fallback value
This could arguably goes in the previous changeset, but I wanted to keep that
previous changeset small to focus more on the user code and the documentation.
Differential Revision: https://phab.mercurial-scm.org/D11687
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}