Automatically answer excuse prompts if stdin is not a TTY

Summary: Fixes T3696. Currently, we abort. If stdin is not a TTY, we should just continue. A script which cares could conceivably run `arc lint` and `arc unit` separately, but it seems unlikely that any script would ever want to fail here.

Test Plan: Ran `echo -n '' | arc diff --create --verbatim` with a lint error and got a revision (D6720).

Reviewers: Firehed, btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3696

Differential Revision: https://secure.phabricator.com/D6721