Synchronize (hosted, git, clustered, SSH) repositories prior to reads

Summary:
Ref T4292. Before we write or read a hosted, clustered Git repository over SSH, check if another version of the repository exists on another node that is more up-to-date.

If such a version does exist, fetch that version first. This allows reads and writes of any node to always act on the most up-to-date code.

Test Plan: Faked my way through this and got a fetch via bin/repository update; this is difficult to test locally and needs more work before we can put it in production.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15757