Address some PHP8 incompatibilities - Arcanist

Summary:
Found a few more issues while playing around with arc.

Refs T13588

Test Plan:

• Run arc look and variants with subcommands
• Run arc diff while not on a git branch

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21870