effectflag: detect when parents changed
Store in effect flag when the parents changed between the predecessor and
its successors.
It can happens with "hg rebase" or "hg grab".
Differential Revision: https://phab.mercurial-scm.org/D539
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}