We want to automatically copy the appropriate mailing list
for review requests to the libunwind repository.
For context, see the proposal and discussion here:
Similar to D40500, I set up a new Diffusion repository with callsign
"UNW" for libunwind:
This explicitly updates libunwind's .arcconfig to point to the new
UNW repository in Diffusion, which will let us use a Herald rule
to copy the correct list on reviews.