Add a character marker to the "IMPLICIT COMMITS" warning in "arc land"

Summary:
Ref T13576. The "implicit commits" prompt in "arc land" shows a list of implicit and non-implicit commits.

The implicit commits are marked with a background color, but this doesn't survive if you copy/paste the output into a support ticket.

Make my life easier by also marking commits so the marker survives copy/paste.

Test Plan: Ran "arc land" with implicit commits, saw a copy-pastable indicator.

Maniphest Tasks: T13576

Differential Revision: https://secure.phabricator.com/D21589