graphlog: use '%' for other context in merge conflict
This lets the user more easily find the commit that is involved in the
conflict, such as the source of `hg update -m` or the commit being
grafted by `hg graft`.
Differential Revision: https://phab.mercurial-scm.org/D8043
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}