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
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.