utils/release: Add merge-git.sh

Summary:
This script allows you to use git to backport a commit to a stable
branch while generating the exact same commit message (ignoring
whitespace) that you would get from using the merge.sh script with svn.

Reviewers: hansw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47760

llvm-svn: 334568