transaction: make journal a private attribute
This attribute tracks the name of the journal file. It is an
implementation detail of the current transaction and therefore
shouldn't be exposed as part of the interface. Let's mark it as
private.
Differential Revision: https://phab.mercurial-scm.org/D4624
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"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}