upgrade: don't perform anything if nothing to do
Before this patch, upgrade will process everything, re-clone all revlogs, write
requirements file again even there is no change to be made.
This patch makes it exit earlier.
Differential Revision: https://phab.mercurial-scm.org/D9695
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"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}