Fix file.line diff formatting.
Fix file.line diff formatting.

Using _splitlines_preserving_trailing_newline() strips off line
endings, mangling the unified_diff output. Switching to splitlines
with the keepends option resolves this issue, while still
preserving trailing newline.

Fixes #41474

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt