search: remove dependency on args in Zoekt subset repo search

Created by: rvantonder

Mechanical change. Semantics preserving.

newIndexedSubsetSearchRequest depends on args (TextParameters) but only needs three values from here. This is an intermediate restructure that makes the function depend only on these three values, and the caller supplies them. It temporarily increases the number of arguments, which will be either removed or condensed in subsequent PRs.

The reason this makes sense is that of these three values, the function really only depends on one runtime value (repos). All of the other values should be validated earlier, or converted to a data type/function that incorporates runtime concerns.