Fix "arc branch" performance regression

Summary: See discussion in T1467. This log logs everything in the repo. The old command was show -s, I just unthinkingly converted it and it doesn't matter for non-Facebook-sized repositories.

Test Plan: Ran "arc branch".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1467

Differential Revision: https://secure.phabricator.com/D2963