Add modern arc config for default "onto" branch

[GMNGeoffrey]

Summary

The config option for this changed in
https://secure.phabricator.com/D21313 (or when that was rolled out).
I'm leaving the old config option, which may be in use by people with
older versions of arc.

[mehdi_amini]

Thanks for figuring it out ;)

.arcconfig

 {
   "phabricator.uri" : "https://reviews.llvm.org/",
   "repository.callsign" : "G",
   "conduit_uri" : "https://reviews.llvm.org/",
   "base": "git:HEAD^",
-  "arc.land.onto.default": "main"
+  "arc.land.onto.default": "main",
+  "arc.land.onto": ["main"]
 }