This is a partial revert of https://chromium-review.googlesource.com/c/527345/
Turns out more people were confused by the new behavior than
expected, so we're returning to "cherry-pick" being the default,
but supporting the collaboration workflow is important, so we're
adding a warning message and support for "reset --hard" behind a
pre-existing flag.

Bug: 723787
Change-Id: Ib6038a42e3bdcc0db93c1f32d759e9ff0e91a065
Reviewed-on: https://chromium-review.googlesource.com/538137
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>