git cl split previously had no way to find out what it would do before
it went ahead and uploaded CLs. Now you can use --dry-run to get it
to print info about each CL it would upload.

Change-Id: If2ac74a89b0a8b30d86819e16ece46f487b1e7c7
Reviewed-on: https://chromium-review.googlesource.com/821336Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Chris Watkins <watk@chromium.org>