Summary: Depends on D20873. Ref T13436. Allow callers to configure "bin/ssh-auth --sshd-key %k" as an "AuthorizedKeysCommand"; if they do, and we recognize the key, emit just that key in the output.
Test Plan:
- Used `git pull` locally, still worked fine.
- Instrumented things, saw the public key lookup actually work and emit a single key.
- Ran without "--sshd-key", got a full key list as before.
Maniphest Tasks: T13436
Differential Revision: https://secure.phabricator.com/D20874