wireprotov2: implement command for retrieving raw store files Implementing shallow clone of the changelog is hard. We want the 4.8 release to have a fast implementation of partial clone in wireprotov2. In order to achieve fast, we can't use deltas for transferring changelog and manifestlog data. Per discussions at the 4.8 sprint, this commit implements a somwwhat hacky and likely-to-be-changed-drastically-or-dropped command in wireprotov2 that facilitates access to raw store files, namely the changelog and manifestlog. Using this command, clients can perform a "stream clone" of sorts for just the changelog and manifestlog. This will allow clients to fetch the changelog and manifest revlogs, stream them to disk (which should be fast), then follow up filesdata requests for files revision data for a particular changeset. Differential Revision: https://phab.mercurial-scm.org/D5134