Have ka-lint with no arguments be a noop, instead of linting everything.
Have ka-lint with no arguments be a noop, instead of linting everything.

Summary: To get the old behavior, do `ka-lint .`.

Test Plan:
I did `git grep -e ka-lint -e runlint.sh` in webapp and saw nobody
calling it with zero args, expecting to lint the entire tree.  In
fact, we don't think anyone will ever lint the entire tree anymore;
it's too slow.

Reviewers: benkraft

Reviewed By: benkraft

Subscribers: kevinb, #infrastructure

Differential Revision: https://phabricator.khanacademy.org/D60004

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