grep: don't look up copy info unless --follow is given
If no --follow was given, then the "copy" variable will become
None. In that case we would still look up the copy information from
the filelog and then ignore it. Let's avoid even looking it up.
Differential Revision: https://phab.mercurial-scm.org/D5620
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}