[polly] Set up .arcconfig to point to new Diffusion PLO repository

[benhamilton]

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:

http://lists.llvm.org/pipermail/cfe-dev/2017-November/056032.html

Similar to D40179, I set up a new Diffusion repository with callsign
"PLO" for polly:

https://reviews.llvm.org/source/polly/

This explicitly updates polly's .arcconfig to point to the new PLO
repository in Diffusion, which will let us use Herald rule H270.

.arcconfig

 {
-  "project_id" : "polly",
+  "repository.callsign" : "PLO",
   "conduit_uri" : "https://reviews.llvm.org/",
   "history.immutable" : true,
   "load" : [
     "utils/arcanist/LitTestEngine"
   ],
   "unit.engine" : "LitTestEngine"
 }