This CL implements the proposal (from the design doc in the cited bug)
to allow `git cl split` to save the generated splitting to a
human-readable and editable text file. This allows users to save the
splitting in case something goes wrong and they need to retry, and also
lets them tweak a generated splitting if they get something mostly, but
not quite, usable.
It also allows users to edit the generated splitting interactively, like
they do for e.g. CL descriptions during `git cl upload`.
Bug: 389069356
Change-Id: I8ae21f2eb7c022ba181ae5af273ce09605b5acec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6235727
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Devon Loehr