phabricator: use .arcconfig for the callsign if not set locally (issue6243) This makes things easier for people working with more than one repository because this file can be committed to each repository. The bug report asks to read <repo>/.arcrc, but AFAICT, that file lives in ~/ and holds the credentials. And we already track an .arcconfig file. Any callsign set globally is still used if that is all that is present, but .arcconfig will override it if available. The idea behind letting the local hgrc override .arcconfig is that the developer may need to do testing against another server, and not dirty the working directory. Originally I was going to just try to read the callsign in `getrepophid()` if it wasn't present in the hg config. That works fine, but I think it also makes sense to read the URL from this file too. That would have worked less well because `readurltoken()` doesn't have access to the repo object to know where to find the file. Supplimenting the config mechanism is less magical because it reports the source and value of the properties used, and it doesn't need to read the file twice. Invalid hgrc files generally cause the program to abort. I only flagged it as a warning here because it's not our config file, not crucial to the whole program operating, and really shouldn't be corrupt in the typical case where it is checked into the repo. Differential Revision: https://phab.mercurial-scm.org/D7934