fix(plugins): noop if config hook returns same config reference

[bluwy]

Description

If the config() hook returns the same config given, Vite currently merges it again, causing the config to merge onto itself, causing unintended behaviour like resolve.conditions duplicating or other array-based fields.

This PR skips the merge if so to prevent unnecessary work, which in most cases shouldn't be a breaking change. Open to discussion though if we think this is nice to have, or we can ignore it for now.

I found this footgun happening in withastro/astro#12287. And searching around, it seems to happen elsewhere:

• https://github.com/MikeBild/serverless-workshop-sveltekit/blob/01a0cb2cec362602e79d34ae0532d7588ae5e6ad/vite.config.js#L10-L20
• https://github.com/zwq8299174/auto-css-plugin/blob/2fe09511aa34c56ffd7caeef1b23abcd4e208037/src/vite-use.js#L9-L13

[patak-dev]

Oh... this makes complete sense. Interesting it wasn't discovered before.