exchangev2: support for calling rawstorefiledata to retrieve raw files This is somewhat hacky. For that I apologize. At the 4.8 Sprint, we decided we wanted to land support in wireprotov2 for doing a partial clone with changelog and manifestlog bootstrapped from a "stream clone" like primitive. This commit implements the client-side bits necessary to facilitate that. If the new server-side command for obtaining raw files data is available, we call it to get the raw files for the changelog and manifestlog. Then we fall through to an incremental pull. But when fetching files data, instead of using the list of a changesets and manifests that we fetched via the "changesetdata" command, we do a linear scan of the repo and resolve the changeset and manifest nodes along with the manifest linkrevs. Differential Revision: https://phab.mercurial-scm.org/D5135