Simplify diff excuses

Summary:
The theory is that lint and unit errors are rare with no false positives.
Our reality is currently different.

This diff removes the question for providing explanation before actually providing it.
I hope that the wording is not confusing.

It also simplifies the code with less copy/paste errors (not me!) potential.

Test Plan:
arc diff on code with lint and unit errors.
Repeat with --excuse ..

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin, meitros

Differential Revision: https://secure.phabricator.com/D2631