R1:661c758ff9d1

Render indent depth changes more clearly

Summary:
Ref T13161. See PHI723. Our whitespace handling is based on whitespace flags like `diff -bw`, mostly just for historical reasons: long ago, the easiest way to minimize the visual impact of indentation changes was to literally use `diff -bw`.

However, this approach is very coarse and has a lot of problems, like detecting `"ab" -> "a b"` as "only a whitespace change" even though this is always semantic. It also causes problems in YAML, Python, etc. Over…