Don't render unchanged files in the diff size action comment

[NullVoxPopuli]

There are a number of PRs where I'm breaking the 65k comment body limit 🙈

[github-actions]

📊 Package size report   -0%↓

File	Before (Size / Brotli)	After (Size / Brotli)
dist/ember-template-compiler.js	769.7 kB / 114.6 kB	-0%↓769.7 kB / -0.05%↓114.5 kB
dist/ember-testing.js	47.9 kB / 8.6 kB	-0.01%↓47.9 kB / -0.06%↓8.6 kB
dist/ember.debug.js	2.1 MB / 321.8 kB	-0%↓2.1 MB / 0.01%↑321.8 kB
dist/ember.prod.js	1.9 MB / 292.3 kB	-0%↓1.9 MB / 0.03%↑292.4 kB
Total (Includes all files)	17.3 MB / 3.2 MB	-0%↓17.3 MB / 0%↑3.2 MB
Tarball size	3.9 MB	-0%↓3.9 MB

_{🤖 This report was automatically generated by pkg-size-action}