When doing presubmit bisects (walking back through git history to see
when a presubmit regression was introduced) it is inconvenient to have
to create a branch at every step, and clean up the branches later. This
change makes having a branch optional, when using --force mode.

Bug: 1309977
Change-Id: I9fb6235620cf6c2e856359d2c25f1ef00c5da554
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3611025Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>