Add callsign to .arcconfig for proper repo identification

Summary:
If you have remotes configured for a branch, `arc` gets confused when trying
to find the configuration for staging areas. This change explicitly sets the
repository callsign in `.arcconfig` which arc uses to look up the appropriate
staging area.

Test Plan: Push a commit and see if it goes to staging properly

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1814

.arcconfig

@@ -1,5 +1,6 @@
 {
   "conduit_uri" : "https://reviews.bitcoinabc.org/",
+  "repository.callsign": "ABC",
   "base" : "git:HEAD^, hg:.^",
   "git.default-relative-commit" : "HEAD^",
   "git:arc.feature.start.default" : "origin/master",