Add repository callsign to arcanist in docs

[rscheff]

Summary

Without it, running "arc diff" comes back to me claiming that it can not figure out the repository:

[root@freebsd ~/freebsd-master/doc]# arc which
REPOSITORY
To identify the repository associated with this working copy, arc followed this process:

    Configuration value "repository.callsign" is set to "rS".

    No repositories matched the query. Check that your configuration is
    correct, or use "repository.callsign" to select a repository explicitly.

This working copy is not associated with any repository.

When the .arcconfig contains this setting, arcanist works without complaining:

[root@freebsd ~/freebsd-master/doc]# arc which
REPOSITORY
To identify the repository associated with this working copy, arc followed this process:

   Configuration value "repository.callsign" is set to "D".

   Found a unique matching repository.

This working copy is associated with the FreeBSD doc repository - subversion repository.

[lwhsu]

ACK, I personally like this, but I also want to note that this also needs some more testing and config change in the current phabricator setup. The old svn repo also used D as callsign and I don't know if changing/reusing it will cause any side effects.

[rscheff]

Looking into rS src .arcconfig, it seems like there, for a long time the callsign was not present - but got added back in with 1906ce9ca7d8 due to the migration to git.

Apparently, that didn't have any detrimental effect (and I didn't notice it in my src arcanist workflow actually).

[rscheff]

Abandoning, as this doesn't seem to get any traction with maintainers of docs