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
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