Prompt users to ignore or abort on untracked files

Summary:
Fixes T7521. This separates things into two prompts and doesn't try to automatically add untracked files.

This also fixes T7512, or at least I can't reproduce it after this change.

Test Plan:

• Ran arc diff in a git directory with untracked files.
• Ran arc diff in a svn directory with untracked files.
• Ran arc diff in a hg directory with untracked files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7512, T7521

Differential Revision: https://secure.phabricator.com/D12258