branch_util: disallow use of 'master'-build branch_util

BUG=None
TEST=local run

Change-Id: I4e26ab15f735eb2375645a63f0887301a932e625
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2684332
Tested-by: Sean Abraham <seanabraham@chromium.org>
Auto-Submit: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Julio Hurtado <juahurta@google.com>
Commit-Queue: Julio Hurtado <juahurta@google.com>
1 file changed