Improve grid layout in "arc branches" at various terminal widths

Summary: Ref T13546. Make "arc branches" use a flexible grid width and try to match the content to the display width in a reasonable way.

Test Plan: Ran "arc branches" at various terminal widths, got generally sensible output.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21365