bundle2: only grab a transaction when 'phase-heads' affect the repository
The next patch will use the 'phase-heads' part to exchange phase data relevant to
the pulled set.
'handlephases' currently acquires a transaction even in case of no-op pull,
which would results in an empty transaction and messing with the existing
journal.
Pass the transaction fetcher to updatephases so it can fetch it if necessary.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}