New Sniff: wp_get_post_revisions memory issues

## What problem would the enhancement address for VIP?
`wp_get_post_revisions` can be problematic because it loads the entire post object for each revision. I've seen this in CLI commands and other places, where you might be looping through a set of posts, and then suddenly have to load the entire post objects for every revision a post has. If those revisions are large and numerous - it quickly fails.

Related trac: https://core.trac.wordpress.org/ticket/34560

## Describe the solution you'd like
We should suggest they only get the IDs for the revisions instead of the whole object:

```
wp_get_post_revisions( $post->ID, array( 'fields' => 'ids' ) );
```

## What code should be reported as a violation?

Anything that doesn't have the fields argument, like:

```
wp_get_post_revisions( $post->ID );
```

## What code should *not* be reported as a violation?

Getting the `ids` field or other fields instead of the whole object.
```
wp_get_post_revisions( $post->ID, array( 'fields' => 'ids' ) );
```