bundle2: only grab a transaction when 'phase-heads' affect the repository...
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.