Remove redundant ["set"] metadata from diff

[josephburnett]

The ["set"] metadata emitted by the diff operations is not strictly necessary. The use of an object implies set semantics and it works fine without it.

jd should stop emitting that metadata (but should continue supporting its explicit use). Makes the simple use cases easier to read.

[josephburnett]

Instead of

@ ["foo",["set"],{}]
- "bar"
+ "baz"

we should emit

@ ["foo",{}]
- "bar"
+ "baz"

because it’s visually cleaner and they do the same thing.