Mapbox style diff testing produce too many changes in diff

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**mapbox-gl-js version**: 0.52.0

**browser**: Chrome

### Steps to Trigger Behavior

1. Load style with deprecated properties (like "is-stroke")
2. Rebuild exactly same style
3. Diff failing on perform update on deprecated properties and rebuilding style from scratch

### Expected Behavior

Style update should not detect any changes and especially failing on deprecated properties

### Actual Behavior

Mapbox setStyle(nextStyle, { diff: true}) fail to in some scenarios when original style contains some deprecated properties, even when style not changed at all

This is happening because of [diffStyle](https://github.com/mapbox/mapbox-gl-js/blob/master/src/style/style.js#L435) takes for a diff this.serialize(), which is a modified ([_loadJson](https://github.com/mapbox/mapbox-gl-js/blob/master/src/style/style.js#L217)) version of the original style, and comparing it with not pre-processed nextState.

This diff produces thousands of changes, even if style not modified at all.

I see a couple of options to fix this issue:

- process nextState and pass serialize version of it into diffStyles method
or
- pass this.stylesheet into diffStyles instead this.serialize(), because this.stylesheet contains original version of previously passed style

Start with diffStyle in src/style/style.js around line 435 and the _loadJson path around line 217, then trace how setStyle(nextStyle, { diff: true }) compares the existing and next styles. Reproduce the deprecated-property case with the provided steps; done means rebuilding an unchanged style produces no changes and does not fail on deprecated properties.

Written by the indexing model from the issue text.