R1:f43355855cb8

Add `bin/files compact` for sharing file data storage

Summary:
Fixes T5912. When we write files, we attempt to share storage if two files have the same content.

In some cases, we may not share storage. Examples include:

- Files migrated with `bin/files migrate` (it's simpler not to try to dedupe them).
- Old files, from before storage was sharable (the mechanism did not exist).
- Files broken by the bug fixed in T5912.

Add a script to compact files by pointing files with the same content hash at…