Only run eslint on 2000 files at a time.
Only run eslint on 2000 files at a time.

Summary:
We were running into "commandline too long" errors on os x!  This
should put a stop to that.

Test Plan: ka-lint javascript

Reviewers: colin, benkraft

Reviewed By: benkraft

Subscribers: kevinb, mattdunnrankin

Differential Revision: https://phabricator.khanacademy.org/D43313

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter