Using a non-absolute ref results in deprecation warnings in the logs
when downstream repos run.

Change-Id: Ib72bda258b7155cd2081d7ddf7112d6b94f4f015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3024691
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>