Fix diff start request options

[Rulexec]

There is:

#[serde(flatten)]
from_generation_id: EncodedOptionalGenerationIdFlatJsonData,
#[serde(flatten)]
to_generation_id: EncodedOptionalGenerationIdFlatJsonData,

We need to remove #[serde(flatten)], because there is conflict of from/to fields, they are both reading generationId field.

[Rulexec]

Fixed https://github.com/anfivewer/diffbelt/blob/76fe2f8eaf99bb91aadedd0108d9484eef2699ff/src/http/routing/routes/diff/start.rs##L27-L32