wireprotov2: define and implement "filedata" command Continuing our trend of implementing *data commands for retrieving information about specific repository data primitives, this commit implements a command for retrieving data about an individual tracked file. The command is very similar to "manifestdata." The only significant difference is that we have a standalone function for obtaining storage for a tracked file. This is to provide a monkeypatch point for extensions to implement path-based access control. With this API available, wire protocol version 2 now exposes all data primitives necessary to implement a full clone. Of course, since "filedata" can only resolve data for a single path at a time, clients would need to issue N commands to perform a full clone. On the Firefox repository, this would be ~461k commands. We'll likely need to implement a file data retrieval command that supports multiple paths. But that can be implemented later. Differential Revision: https://phab.mercurial-scm.org/D4490