R1:156adccef0ad

Fix an issue where "bin/differential migrate-hunk" could decompress data

Summary:
Fixes T12986. I caught this bug in the changes from D18584: when we moved a large hunk to file storage, we would decompress it but keep the "deflated" flag. This could cause confusion when loading it later. I missed this in testing since I wasn't exhaustive enough in checking hunks and didn't run into a compressed one.

Instead of compressing on `save()`, compress during the normal workflow.

We currently never advise users…