rebase: define base in only place in defineparents()
Just a little refactoring to prepare for the next patch.
Differential Revision: https://phab.mercurial-scm.org/D7906
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"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}