Summary:
This is a follow-up of D4788756 and makes it easier to run `arc unit` or
`arc lint`.
Test Plan:
Run `arc lint` and `arc unit` without sourcing `hg-dev` first. And also rename
`facebook-hg-rpms` to make sure it aborts if `hg-dev` cannot be found.
Reviewers: simpkins, #sourcecontrol, simonfar
Reviewed By: simonfar
Subscribers: net-systems-diffs@fb.com, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4831818
Signature: t1:4831818:1491382785:35f6537722502d3c63cb65cf7129564b26903201
