R1:66fa59d04d7c

Fix an issue where migrating files could prematurely destroy duplicates

Summary:
Fixes T5912. When migrating files, we try to clean up the old data. However, this code isn't aware of reference counting, and unconditionally destroys the old data.

For example, if you migrate files `F1` and `F2` and they have the same data, we'll delete the shared data when we migrate `F1`. Then you'll get an error when you migrate `F2`.

Since this only affects duplicate files, it primarily hits default profile pictures…