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
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