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

Summary:
Depends on D19370. See T13124. See PHI549. The particular install in PHI549 migrated a large amount of data via the fallback hunk migration script, which does not compress hunks.

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.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19371