In "arc branch" output, sort branches updated in the same second by name

Summary:
Ref T13546. The new "arc land" workflow can rebase several branches per second. With branches like "feature1", "feature2", etc., this leads to out-of-order listing in "arc branch".

When two branches would otherwise sort to the same position, sort them by name.

Test Plan: Ran "arc branch" after a cascading rebase by "arc land", saw "land5", "land7", "land8", etc., instead of an arbitrary order.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21316