Allow alternate_target_roots to specify an empty collection

Review Request #3216 - Created Dec. 7, 2015 and submitted

Nick Howard (Twitter)
benjyw, jsirois, peiyu, zundel
For tasks like test-changed, it makes sense for them to become no-ops when there are no changed targets. This patch allows alternates to be set to an empty set so that test-changed will run no tests when there are no changed targets rather than running all the tests.

Wrote a test case for the desired behavior that failed, then changed the implementation to make it pass. Running CI on travis @ CI passed.

Nick Howard (Twitter)
Patrick Lawson
Eric Ayers
Nick Howard (Twitter)
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted at

Benjy Weinberger
Ship It!