R1:e3de7d09c0d8

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