Just checked with the old and new versions of Title_09.txt and you're right, --word-diff does the right thing. It echoes all the changed lines to the terminal, but it only marks up (and colors, in color mode) the changed words:

          [-Section 2 of act-]{+Act+} July 30, 1947, ch. 392, {+Sec. 2,+} 61 Stat. 674, provided that

Wonder if there's a way to enable that behavior on GitHub? And/or to generate repository activity statistics based on changed words rather than changed lines?