(stable) Fix an unusual issue with intradiff highlighting of files with uncommon end-of-file modifications

Summary:
Fixes T13539. See that task for discussion and a reproduction case.

This algorithm currently counts "\ No newline at end of file" lines as though they were normal source lines. This can cause offset issues in the rare case that a diff contains two of these lines (for each side of the file) and has changes between them (because the last line of the file was modified between the diffs).

Instead, don't count "\" as a display line.

Test Plan:

• See T13539 and PHI1740.
• Before: got fatals on the "wild" diff and the synthetic simplified version.
• After: clean intradiff rendering in both cases.

Maniphest Tasks: T13539

Differential Revision: https://secure.phabricator.com/D21267