Initial implementation of an async branch dropdown for Revert and Cherry Pick.

app/assets/stylesheets/framework/modal.scss

 .modal-body {
   position: relative;
-  overflow-y: auto;
   padding: 15px;
   .form-actions {

app/controllers/projects/branches_controller.rb

@@ -15,6 +15,13 @@ class Projects::BranchesController < Projects::ApplicationController
       diverging_commit_counts = repository.diverging_commit_counts(branch)
       [memo, diverging_commit_counts[:behind], diverging_commit_counts[:ahead]].max
     end
+
+    respond_to do |format|
+      format.html
+      format.json do
+        render json: @repository.branch_names.to_json
+      end
+    end
   end
   def recent

app/views/projects/commit/_change.html.haml

@@ -17,7 +17,14 @@
           .form-group.branch
             = label_tag 'target_branch', target_label, class: 'control-label'
             .col-sm-10
-              = select_tag "target_branch", project_branches, class: "select2 select2-sm js-target-branch"
+              .dropdown
+                = dropdown_toggle @project.default_branch, { toggle: "dropdown", selected: @project.default_branch, ref: @ref, refs_url: branches_namespace_project_path(@project.namespace, @project) }, { toggle_class: "js-project-refs-dropdown js-target-branch" }
+                .dropdown-menu.dropdown-menu-selectable
+                  = dropdown_title "Switch branch"
+                  = dropdown_filter "Search branches"
+                  = dropdown_content
+                  = dropdown_loading
+
               - if can?(current_user, :push_code, @project)
                 .js-create-merge-request-container
                   .checkbox