R1:f2938bacd9c4

Generalize SSH passthru for repository hosting

Summary:
Ref T2230. In Git, we can determine if a command is read-only or read/write from the command itself, but this isn't the case in Mercurial or SVN.

For Mercurial and SVN, we need to proxy the protocol that's coming over the wire, look at each request from the client, and then check if it's a read or a write. To support this, provide a more flexible version of `passthruIO`.

The way this will work is:

- The SSH IO channel is wrapped in a…