Make callsigns optional in arcanist

Summary:
Remove couple of references to callsigns:

• arc which now prints repository name
• getShouldAmend() can now use new format of commit name

a quick git-grep looks like the remaining references are all about repository.callsign config.

Ref T4245

Test Plan:

• arc which on a repository with no callsign
• trigger requireCleanWorkingCopy(), see both "Do you want to amend this change" and "Do you want to create a new commit" prompts.
• fire this diff with new code.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15472