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