Tweaked performance of Issue#related_branches

Requesting the branch names of a repository works even when it's empty,
thus there's no need to explicitly check for an empty repository.
Removing this check cuts down the amount of Git operations which in turn
cuts down request timings a bit. The regular expression used to compare
branches was also moved out of the loop so it's created only once.

app/models/issue.rb

@@ -105,9 +105,9 @@ class Issue < ActiveRecord::Base
   end
 
   def related_branches
-    return [] if self.project.empty_repo?
-    
-    self.project.repository.branch_names.select { |branch| branch.end_with?("-#{iid}") }
+    project.repository.branch_names.select do |branch|
+      branch.end_with?("-#{iid}")
+    end
   end
 
   # Reset issue events cache

spec/models/issue_spec.rb

@@ -135,7 +135,7 @@ describe Issue, models: true do
       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).to eq([subject.to_branch_name])
     end
   end