Add ssh debug info for mesos git update commands.
Add ssh debug info for mesos git update commands.

Summary:
Use "ssh -v" for git update commands to help debug
why mesos slaves sometimes fail on those commands

Test Plan: Tested the commands updating my server repo. We can test this in a staging mesos instance.

Reviewers: brhodes

Reviewed By: brhodes

Subscribers: changesbot, mkedia, vishal

Differential Revision: https://tails.corp.dropbox.com/D119888

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