context: add workingfilectx.markcopied
With in-memory merge, copy information needs to be stored in-memory, not in the
dirstate.
To make this transition easy, move the existing dirstate-based approach to
workingfilectx; that way, other implementations can choose to store it
somewhere else.
Differential Revision: https://phab.mercurial-scm.org/D1106
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}