test-copies: add a test updating file content while salvaging it
A deleted file is brought back during a merge. Its content is changed in the
same go.
This reveal some issue with the upgrade code.
Differential Revision: https://phab.mercurial-scm.org/D10088
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"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}