wireprotov2: support exposing linknode of file revisions When supporting shallow file storage, clients may fetch file revisions by changeset. But they may not readily know which changeset introduced a specific file revision. The "linknode" is used to record which changeset introduces which file revision. This commit teaches the "filedata" and "filesdata" wire protocol commands to expose the linknode for file revisions. The implementation is likely wrong when hidden changesets are in play, since the linknode may refer to a hidden changeset. We can deal with this problem later. Differential Revision: https://phab.mercurial-scm.org/D5167