Don't pass "No newline at end of file." annotations to DocumentEngines as literal diff text

Summary: See PHI1707, which has a Jupyter notebook which fails to diff nicely when modified. The root cause seems to be that the document does not end in a newline.

Test Plan: Applied patch, diffed the file, got a Jupyter diff out of it.

Differential Revision: https://secure.phabricator.com/D21159