Review request https://reviews.reviewboard.org/r/10164/ landed on

release-4.0.x, but the underlying issue was introduced on release-3.0.x.

This patch is a backport of that patch. The original commit message

follows:
We had a unit test that depended on the specific version of patch

installed. Newer versions of patch will accept a diff that contains no

hunks, whereas older versions will fail with a message saying that the

input contained only garbage.
We now have unit tests that check the correct behaviour happens in both

cases by replacing our call to patch.

Ran unit tests.