[lib] Return result of mergeNewMessages(...) immediately for fetchMessagesBeforeCursor.success`/fetchMostReecentMessages.success action types

Summary:
We were previously destructuring the return value of mergeNewMessages(...) and passing the destructured variables into eg assertMessageStoreMessagesAreEqual(...).

But, we're not doing that anymore so we can just return directly and cut a few lines.

Depends on D4336

Test Plan: flow, close reading

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4337