only send format editings when necessary

[tw4452852]

If the original document is same as the formatted one, there's no need to send the unchanged
document's content back which will make the client confused.

[SuperAuguste]

This change is unnecessary and will also brick some clients that will wait for a format response they will never receive. The other alternatives are to send an empty or error response, but those would break clients too, thus the status quo is better. :)

[tw4452852]

Hi @SuperAuguste

According to the spec, if there's nothing change, we could return null result in response, what do you think?

[SuperAuguste]

That would work, but I still do not see why this is a necessary change. Is there an example of where the status quo behavior is problematic?

[tw4452852]

I found this issue with my acme editor, it will trigger formatting on saving, but every time it gets a non-empty response, it will mark the current file dirty and format the content although the formatted one and original one are the same. It's kind of annoying and I didn't encounter this problem when using another language server (e.g. gopls, rust-analyzer).

[SuperAuguste]

I see! If you make format return a null instead of nothing, I'll merge this PR. Thanks!

[tw4452852]

Hi @SuperAuguste

I just rebased the patch onto the latest master and change the behavior to respond with a null result when no changes.

[SuperAuguste]

LGTM, thank you so much for your contribution! Hopefully this fixes the problems you and others were facing with zls in acme. :)

[tw4452852]

Yeah, feel happy if my fix could help others.