Tweak checking branches in Project#open_branches

This changes 4 things:

1. Project#protected_branches_names has been renamed to
   Project#protected_branch_names.

2. Project#open_branches uses a Set for the branch names as checking
   values in a Set is faster than checking values in a (large) Array.

3. Some redundant code in Project#open_branches has been removed.

4. Project#protected_branch_names now uses #pluck instead of #map,
   removing the need for loading entire DB records into memory.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.8.0 (unreleased)
+  - Project#open_branches has been cleaned up and no longer loads entire records into memory.
   - Make build status canceled if any of the jobs was canceled and none failed
   - Remove future dates from contribution calendar graph.
   - Use ActionDispatch Remote IP for Akismet checking

app/models/project.rb

@@ -735,19 +735,17 @@ class Project < ActiveRecord::Base
   end
 
   def open_branches
-    all_branches = repository.branches
+    # We're using a Set here as checking values in a large Set is faster than
+    # checking values in a large Array.
+    protected_set = Set.new(protected_branch_names)
 
-    if protected_branches.present?
-      all_branches.reject! do |branch|
-        protected_branches_names.include?(branch.name)
-      end
+    repository.branches.reject do |branch|
+      protected_set.include?(branch.name)
     end
-
-    all_branches
   end
 
-  def protected_branches_names
-    @protected_branches_names ||= protected_branches.map(&:name)
+  def protected_branch_names
+    @protected_branch_names ||= protected_branches.pluck(:name)
   end
 
   def root_ref?(branch)