JSON: list only changed words of changed comments, strings and text lines

[jrosiek]

In the JSON output, a changed line of code lists only the tokens that changed, but a changed comment, string or text line lists every word on it, the unchanged ones included. difft already knows which words really changed (NovelWord vs UnchangedPartOfNovelItem) and uses that in its own display, emphasising only the changed words; the JSON drops the distinction, because every novel match kind is serialised the same way.

This leaves those unchanged words out of changes, as unchanged code tokens already are. JSON consumers can then show intraline changes for comments, strings and text the same way difft displays them, and the meaning of changes is the same for code and text: what changed on the line.

It covers the line-oriented fallback too (unsupported languages, DFT_BYTE_LIMIT, DFT_GRAPH_LIMIT, DFT_PARSE_ERROR_LIMIT), since that path produces the same match kinds.

Example, a changed Rust comment:

// Compute the total price of the order including tax.
// Compute the total cost of the order excluding tax.

changes contents before (each side lists all 21 tokens of the line):

lhs: "/", "/", " ", "Compute", " ", "the", " ", "total", " ", "price", " ", "of", ... "including", " ", "tax", "."
rhs: "/", "/", " ", "Compute", " ", "the", " ", "total", " ", "cost", " ", "of", ... "excluding", " ", "tax", "."

after:

lhs: "price", "including"
rhs: "cost", "excluding"

A line inside a changed comment block that is the same on both sides now has an empty changes list, like an unchanged code line within a hunk.

Addresses #658.

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