Mark unreachable commits handles as "closed"

Summary:
Ref T9028. Mostly, this gives them a strikethru style.

(I think this is probably the right definition of "closed" for commits. Another definition might be "audited", but I don't think completing audits really "closes" a commit.)

Test Plan: {F1689662}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16135