Read only stdout in LinesOfALargeExecFuture

Summary:
We read all the output before throwing so stderr is empty.
I hit this issue in D5738.

Test Plan: Searched for invalid regexp in D5738.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5767
Add a convenience method for instantiating concrete subclasses of some abstract ancestor

Summary: This pattern occurs fairly often in Phabricator; add a helper method.

Test Plan: Used in D5756.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5757