Summary:
This allows a third-party tool like Buck report more detail on what a sparse
command actually achieved.
Test Plan: Run the test suite.
Reviewers: #sourcecontrol, durham, rmcelroy, ikostia
Reviewed By: ikostia
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4962550
Tasks: 17150508
Tags: autosparse
Signature: t1:4962550:1493307403:d14e16d079121688a4eb69f0d69cd02ab55c4936
