Use $EDITOR to prompt users when creating a new commit out of dirty working copy changes

Summary:
Fixes T7344.
Currently, we use phutil_console_prompt(), which isn't a very good editor. Use the real $EDITOR instead.

100% of the logic here was also a gigantic mess. Clean it up.

Test Plan: Will update in a second with console output from this run.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7344

Differential Revision: https://secure.phabricator.com/D11843