Add an optional protocol log to git SSH workflows

Summary:
Ref T8093. Support dumping the protocol bytes to a side channel logfile, as a precursor to parsing the protocol and rewriting protocol frames to virtualize refs.

The protocol itself is mostly ASCII text so the raw protocol bytes are pretty comprehensible.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T8093

Differential Revision: https://secure.phabricator.com/D20380