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.

phases: move the binary decoding function in the phases module We move the decoding function near the encoding one in a place where they can be reused in other place (current target, 'exchange.py').

phases: move binary encoding into a reusable function We want to use binary phases for pushing and pulling. We extract the encoding function out of the bundle2 module first.