Follow-up from "Branches: Branch item update"

The following discussion from !119637 (merged) should be addressed:

• @OmarQunsulGitlab started a discussion:

  (not-blocking) As we tend usually to query the database objects using finders, have you considered reusing the MergeRequestFinder instead? You can maybe add a parameter to the finder to use the newly introduced scope by_sorted_source_branches, to make it available for other callers.

  I don't want this to block you

• Or !119637 (comment 1390938916)

This isn't a blocker, and not quite a nit, but I wonder if this logic would live on MergeRequests::BaseService comfortably? We're already doing some semi-related searches there.. 🤔

Proposal

Update the code to use MergeRequestFinder instead of requesting the list of merge requests from the controller.