R1:b5f23b023e3c

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…