Display informative message when arc launches an editor
Update PhutilInteractiveEditor to allow specifying a "task message" which will be displayed just prior to launching the user's editor.
Test Plan: I ran several arc diff commands in varying states to invoke my editor and verified that it printed out the text indicating that my editor was being launched.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Maniphest Tasks: T3271
Differential Revision: https://secure.phabricator.com/D21700