`git cl split` splits a branch into smaller branches and
uploads CLs.

Change-Id: Ic41cdabdd6241008ff48766e31a8d9d07995f2b0
Reviewed-on: https://chromium-review.googlesource.com/439706
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>