exchangev2: use filesdata filesdata is a more efficient mechanism for bulk fetching files data for a range of changesets. Let's use it in exchangev2. With this change, a client performing a full clone of mozilla-unified transmits substantially fewer bytes across the wire: before: 139,124,863 bytes sent after: 20,522,499 bytes sent The bulk of the remaining bytes is likely the transfer of ~1M nodes for changesets and manifests. We can eliminate this by making requests in terms of node ranges instead of explicit node lists... Differential Revision: https://phab.mercurial-scm.org/D4982