`git cl split` has the potential to do a lot of operations at once,
which is problematic if it didn't end up splitting things in a way
the user approves of. This changes it to print a summary of the
splitting, and asking the user to review it briefly before proceeding
with the upload.
Bug: 389069356, 40642562, 40269201
Change-Id: I90bead0147badbaa3afcce2264d805ae498f101c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6163905
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Andy Perelson <ajp@google.com>
