git-llvm: Add option to push to Github with SSH

Summary:
For users with 2FA Github password challenge can be rather cumbersome
and I personally use key based authentication through SSH. This fixes
the SSH feature that was present and exposes it as a command line
option.

Reviewers: tstellar

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71534