Indicate that a final newline was added in --diff (#1897) (#1897)

Fixes: #1662

Work-around for https://bugs.python.org/issue2142



The test has to slightly mess with its input data, because the utility
functions default to ensuring the test data has a final newline, which
defeats the point of the test.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>