Regression: Text files appear as all modified when opened

A red line is displayed for the whole file. When saved, whitespace is cleaned for the whole file even if "In entire document" is unchecked.

This happens for files that use the generic highlighter (plain text, xml etc...)

Regression introduced in 113c5600c909791ddfea3d424a41d982dcdda6b7

Proposed fix in https://codereview.qt-project.org/63017