Extracted ProtectableDropdown to clean up Project#open_branches

Makes it clear this is only used in dropdowns, instead of cluttering up Project class. Since we only care about branch names, it is also possible to refactor out a lot of the set/reject logic.

A benchmark on Array/Set subtraction favoured using Arrays. This was with 5000 ‘branches’ and 2000 ‘protections’ to ensure a similar comparison to the commit which introduced using Set for intersection.

Comparison:
   array subtraction:      485.8 i/s
     set subtraction:      128.7 i/s - 3.78x slower

1 files changed, 0 insertions, 9 deletions

diff --git a/app/models/project.rb b/app/models/project.rb
index 970de324a5b..4175bfab0a9 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -865,15 +865,6 @@ class Project < ActiveRecord::Base
     @repo_exists = false
   end
 
-  # Branches that are not _exactly_ matched by a protected branch.
-  #TODO: Move to Protections::BranchMatcher.new(project).unprotecte
-  def open_branches
-    exact_protected_branch_names = protected_branches.reject(&:wildcard?).map(&:name)
-    branch_names = repository.branches.map(&:name)
-    non_open_branch_names = Set.new(exact_protected_branch_names).intersection(Set.new(branch_names))
-    repository.branches.reject { |branch| non_open_branch_names.include? branch.name }
-  end
-
   def root_ref?(branch)
     repository.root_ref == branch
   end