Refactor `Issue#related_branches`

- Previously, the controller held the logic to calculate
  related branches, which was:

  `<branches ending with `issue.iid`> - <branches with a merge request referenced in the current issue>`

- This logic belongs in the `related_branches` method, not in the
  controller. This commit makes this change.

- This means that `Issue#related_branches` now needs to take a `User`.
  When we find the branches that have a merge request referenced in the
  current issue, this is limited to merge requests that the current user
  has access to.

- This is not directly related to #14566, but is a related refactoring.

1 files changed, 2 insertions, 1 deletions

diff --git a/spec/models/issue_spec.rb b/spec/models/issue_spec.rb
index 15052aaca28..f33b705810e 100644
--- a/spec/models/issue_spec.rb
+++ b/spec/models/issue_spec.rb
@@ -192,10 +192,11 @@ describe Issue, models: true do
 
   describe '#related_branches' do
     it "selects the right branches" do
+      user = build(:user)
       allow(subject.project.repository).to receive(:branch_names).
         and_return(["mpempe", "#{subject.iid}mepmep", subject.to_branch_name])
 
-      expect(subject.related_branches).to eq([subject.to_branch_name])
+      expect(subject.related_branches(user)).to eq([subject.to_branch_name])
     end
   end