Use more modern detection of repositories in "arc close-revision"

Summary:
Fixes T4603. We fire `arc close-revision --finalize` implicitly from `arc land`, which may close a corresponding Differential revision.

We want to close if the repository is not present in Phabricator (i.e., we'll never be able to close in response to the commit message, since we'll never see it). Historically, we used Arcanist Project -> "Tracked" to make this determination. Instead, just check if the working copy is associated with a repository. This is simpler, easier, and works better.

Test Plan: Ran `arc close-revision --finalize`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4603

Differential Revision: https://secure.phabricator.com/D8523