rhg: sort unsupported extensions in error message
This caused some flakiness in test output, and is also just better for users.
Differential Revision: https://phab.mercurial-scm.org/D12389
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}