filemerge: fix crash when using filesets in [partial-merge-tools]
Without this patch, you'd get `mercurial.error.ProgrammingError: fileset
expression with no context`.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}