comark/diff/block-count · Give every ::diff two code blocks, before and after

A ::diff reads its first code block as the file before the change and its second as the file after. With one block there's nothing to compare; leave the other empty for a file that's added or deleted. A third block is dropped.

Incorrect

::diff
```ts [src/ducks.ts]
const meals = 2
```
::

lint-comark example.mdexample.md:1:1 comark/diff/block-countdiff "src/ducks.ts": needs exactly two code blocks, before and after (got 1).Add the missing one; leave it empty for a file that's added or deleted.✖ 1 problem

Correct

::diff
```ts [src/ducks.ts]
```
```ts
const meals = 2
```
::

lint-comark example.md✔ No problems