Add an "--all" flag to "bin/differential migrate-hunk"

Summary:
Depends on D19369. Ref T13120. Add a flag to migrate every hunk.

This isn't terribly useful on its own, but I'm going to add an --auto flag next so that you can run --auto --all to migrate hunks to the preferred hunk format.

Test Plan: Ran bin/differential migrate-hunk --all --to text.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120

Differential Revision: https://secure.phabricator.com/D19370