Add some facilities to work with a git monorepo (experimental setup)

Add some facilities to work with a git monorepo (experimental setup)

Add a new script in llvm/utils/git-svn/. When present in the $PATH,
it enables a `git llvm` command. It is providing at this
point only the ability to push from the git monorepo: `git llvm push`.
It is intended to evolves with more features, for instance I plan on
features like `git llvm show r284955` to help working with sequential
revision numbers.
The push feature is taken from Justin Lebar's script available here:
https://github.com/jlebar/llvm-repo-tools/

Differential Revision: https://reviews.llvm.org/D26334

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286138 91177308-0d34-0410-b5e6-96231b3b80d8