Summary:
`number_format()` returns a string, so if passed a number greater than 999 by default it will add commas, which parsed by %d will only return the 1000's delimter.
```echo sprintf("%d", number_format(1000)); // Outputs 1
echo sprintf("%s", number_format(1000)); // Outputs 1,000
Test Plan: Looked at repos with over 999 commits.
Reviewers: epriestley, chad
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5913
9c87ba8b09