pull: pre-filter remote phases before moving local ones
We were relying on the phase internals to filter out redundant phase
information from remove. However as we plan to integrate phase
movement inside the transaction, we want to avoid useless transaction
creation on no-op pulls.
Therefore we filter out all the information that already matches the current
repository state. This will let us create a transaction only when there is
actual phase movement needed.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.