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, which are the most numerous duplicate files on most installs.

Test Plan:
  - Verified that the theory was correct by uploading two copies of a file and migrating the first one, before applying the patch. The second one's data was nuked and it couldn't be migrated.
  - Applied patch.
  - Uploaded two copies of a new file, migrated the first one (no data deletion), migrated the second one (data correctly deleted).
  - Uploaded two copies of another new file, `bin/remove destory'd` the first one (no data deletion), then did it to the second one (data correctly deleted).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5912

Differential Revision: https://secure.phabricator.com/D10312