Export gitiles commit metadata from all builds in a parent/child relationship such that Milo knows how to group these builds together. This will be the commit of the manifest repo from which the parent and child builds all start to build (before cherry-picking).

Milo uses this information to understand how to show concurrently scheduled builds when rendering the Milo waterfall: https://ci.chromium.org/p/chrome/g/chromium.chrome/console