git cl format can't accept branch name as argument as those are reserved
for files that need to be formatted. This CL clarifies what user needs
to do in case git cl is not able to determine upstream branch.

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

Bug: 832295
Change-Id: If6ff517d633b7e70a33dff48f3779f82df30f214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2571838Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>