Consider allowing individual downstreams to opt-in to `git push`ing template update directly

[jlebon]

Having repo template changes propagate to lots of PRs can get noisy. Some upstream may prefer relying on team approval in this repo when the templates are edited with the understanding that changes will be automatically pushed downstream.

[bgilbert]

One alternative is to let changes accumulate in the PRs, and merge them only when the changes are actually needed (i.e. before release). #13 is intended to help with this by removing PRed-but-not-yet-merged downstream changes from the rendered diff upstream, reducing the incentive to merge the downstream PRs quickly.