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 the same file contnet.

In the particular case of files broken by the bug in T5912, we know the hash of the file's content and will only point them at a file that we can load the data for, so this fixes them.

Compaction is not hugely useful in general, but this script isn't too complex and the ability to fix damage from the bug in T5912 is desirable. We could remove this capability eventually.

Test Plan:
  - Ran `files compact --all --dry-run` and sanity checked a bunch of the duplicates for actually being duplicates.
  - Migrated individual files with `files compact Fnnn --trace` and verified the storage compacted and all files survived the process.
  - Verified unused storage was correctly destroyed after removing the last reference to it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5912

Differential Revision: https://secure.phabricator.com/D10327