search: Remove intermediate allocation

[marcocondrache]

Release Notes:

• N/A

[osiewicz]

Eh, we're not really avoiding an allocation in one of those cases (calling get_matches still allocates). If you're really after reducing allocations, you can do so without altering the API of update_match_index with some finnickery inside of that function.
45633.patch

[marcocondrache]

@osiewicz That patch is incorrect. highlight_matches can’t be called from inside the entity update because contexts are of different types.

Also, the selection-change path is only triggered by user clicks/selections, whereas entity_changed is invoked much more frequently. For context, entity_changed gets called hundreds of times during large searches.

Because of that, update_match_index ends up doing unnecessary allocations when it could instead borrow the already-cloned data from entity_changed.