Only show branches for revert / cherry-pick

Tags are immutable, so we can't add a commit to either revert or
cherry-pick another commit to them.

CHANGELOG

@@ -8,6 +8,7 @@ v 8.9.0 (unreleased)
   - Bump rouge to 1.11.0
   - Make EmailsOnPushWorker use Sidekiq mailers queue
   - Fix wiki page events' webhook to point to the wiki repository
+  - Don't show tags for revert and cherry-pick operations
   - Fix issue todo not remove when leave project !4150 (Long Nguyen)
   - Allow customisable text on the 'nearly there' page after a user signs up
   - Bump recaptcha gem to 3.0.0 to remove deprecated stoken support

app/helpers/branches_helper.rb

@@ -14,4 +14,8 @@ module BranchesHelper
     ::Gitlab::GitAccess.new(current_user, project).can_push_to_branch?(branch_name)
   end
+
+  def project_branches
+    options_for_select(@project.repository.branch_names, @project.default_branch)
+  end
 end

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

@@ -17,7 +17,7 @@
           .form-group.branch
             = label_tag 'target_branch', target_label, class: 'control-label'
             .col-sm-10
-              = select_tag "target_branch", grouped_options_refs, class: "select2 select2-sm js-target-branch"
+              = select_tag "target_branch", project_branches, class: "select2 select2-sm js-target-branch"
               - if can?(current_user, :push_code, @project)
                 .js-create-merge-request-container
                   .checkbox

spec/features/projects/commits/cherry_pick_spec.rb

@@ -16,6 +16,7 @@ describe 'Cherry-pick Commits' do
     it do
       visit namespace_project_commit_path(project.namespace, project, master_pickable_commit.id)
       find("a[href='#modal-cherry-pick-commit']").click
+      expect(page).not_to have_content('v1.0.0') # Only branches, not tags
       page.within('#modal-cherry-pick-commit') do
         uncheck 'create_merge_request'
         click_button 'Cherry-pick'