Avoiding 'master' as a local branch name per naming guidelines hits this
special case and won't allow upload of child branches. Even though more
comprehensive efforts (crbug.com/1095735) have yet to decide on an
official preferred main branch name this change is small and unlikely
to break things (I hope).

This supports people using a "git pull" "merge" workflow.

Change-Id: I8aca044aeb4178c6671e3a1590617f666505b42e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2377189Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Glen Robertson <glenrob@chromium.org>