In 1-up source diffs, retain the "No newline at end of file" on "\" lines

Summary:
See PHI1839. Currently, the "No newline at end of file" text is dropped in the 1-up diff view for changes that affect a file with no trailing newline.

Track it through the construction of diff primitivies more carefully.

Test Plan: {F7695760}

Differential Revision: https://secure.phabricator.com/D21433