[polly] Set up .arcconfig to point to new Diffusion PLO repository
Summary: We want to automatically copy the appropriate mailing list
for review requests to the polly repository.
For context, see the proposal and discussion here:
Similar to D40179, I set up a new Diffusion repository with callsign
"PLO" for polly:
This explicitly updates polly's .arcconfig to point to the new C
repository in Diffusion, which will let us use Herald rule H270.