This option modifies default branch of desired repository.

R=ehmaldonaldo@chromium.org, gavinmak@google.com

Change-Id: I1e20dc8d333c4301eacffff5049e3a98c3d59f75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2575884Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>