Update git-llvm script to push to GitHub

Summary:
Note: This patch should not be pushed until SVN has become read-only.
It should be the first patch committed directly to GitHub.

This patch updates git-llvm to check for merge commits and then push
changes to GitHub if none are found. All logic related to SVN has been
removed.

Reviewers: jyknight

Subscribers: lenary, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67772