We want to automatically copy the appropriate mailing list
for review requests to the openmp repository.
For context, see the proposal and discussion here:
Similar to D40179, I set up a new Diffusion repository with callsign
"OMP" for OpenMP:
This explicitly updates openmp's .arcconfig to point to the new
OMP repository in Diffusion, which will let us use Herald rule H272
to automatically subscribe openmp-commits to review requests.