Add an "--auto" flag to "bin/differential migrate-hunk"
Add a mode to bin/differential migrate-hunk that amounts to "compress all the hunks which would benefit from compression".
Test Plan: Ran bin/differential migrate-hunk with --auto, --all, --to, --id, and --dry-run in various mixtures. Forced a bunch of hunks to raw ("byte") format, saw it cleanly upgrade them to compressed ("gzde") format.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19371