[EE] Refactor branch chooser in issuable form

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/shared/issuable/_approvals.html.haml

+- issuable = local_assigns.fetch(:issuable)
+- form = local_assigns.fetch(:form)
+
 - return unless issuable.is_a?(MergeRequest)
 - return unless issuable.requires_approve?
+
 - approvals_required = issuable.target_project.approvals_before_merge
+
 .form-group
-  = f.label :approvals_before_merge, class: 'control-label' do
+  = form.label :approvals_before_merge, class: 'control-label' do
     Approvals required
   .col-sm-10
-    = f.number_field :approvals_before_merge, class: 'form-control', value: approvals_required
+    = form.number_field :approvals_before_merge, class: 'form-control', value: approvals_required
     .help-block
       Number of users who need to approve this merge request before it can be accepted.
       If this isn't greater than the project default (#{pluralize(approvals_required, 'user')}),
       then it will be ignored and the project default will be used.
 .form-group
-  = f.label :approver_ids, class: 'control-label' do
+  = form.label :approver_ids, class: 'control-label' do
     Approvers
   .col-sm-10
     - author = issuable.author || current_user

app/views/shared/issuable/_form.html.haml

@@ -40,32 +40,9 @@
       title: 'Moving an issue will copy the discussion to a different project and close it here. All participants will be notified of the new location.' }
         = icon('question-circle')
 
-= render 'shared/issuable/approvals', issuable: issuable, f: form
+= render 'shared/issuable/approvals', issuable: issuable, form: form
 
-- if issuable.is_a?(MergeRequest) && !issuable.closed_without_fork?
-  %hr
-  - if @merge_request.new_record?
-    .form-group
-      = form.label :source_branch, class: 'control-label'
-      .col-sm-10
-        .issuable-form-select-holder
-          = form.select(:source_branch, [@merge_request.source_branch], { }, { class: 'source_branch select2 span2', disabled: true })
-  .form-group
-    = form.label :target_branch, class: 'control-label'
-    .col-sm-10
-      .issuable-form-select-holder
-        = form.select(:target_branch, @merge_request.target_branches, { include_blank: true }, { class: 'target_branch select2 span2', disabled: @merge_request.new_record?, data: {placeholder: "Select branch"} })
-      - if @merge_request.new_record?
-         
-        = link_to 'Change branches', mr_change_branches_path(@merge_request)
-  - if @merge_request.can_remove_source_branch?(current_user)
-    .form-group
-      .col-sm-10.col-sm-offset-2
-        .checkbox
-          = label_tag 'merge_request[force_remove_source_branch]' do
-            = hidden_field_tag 'merge_request[force_remove_source_branch]', '0', id: nil
-            = check_box_tag 'merge_request[force_remove_source_branch]', '1', @merge_request.force_remove_source_branch?
-            Remove source branch when merge request is accepted.
+= render 'shared/issuable/form/branch_chooser', issuable: issuable, form: form
 
 - is_footer = !(issuable.is_a?(MergeRequest) && issuable.new_record?)
 .row-content-block{class: (is_footer ? "footer-block" : "middle-block")}

app/views/shared/issuable/form/_branch_chooser.html.haml

+- issuable = local_assigns.fetch(:issuable)
+- form = local_assigns.fetch(:form)
+
+- return unless issuable.is_a?(MergeRequest)
+- return if issuable.closed_without_fork?
+
+%hr
+- if issuable.new_record?
+  .form-group
+    = form.label :source_branch, class: 'control-label'
+    .col-sm-10
+      .issuable-form-select-holder
+        = form.select(:source_branch, [issuable.source_branch], {}, { class: 'source_branch select2 span2', disabled: true })
+.form-group
+  = form.label :target_branch, class: 'control-label'
+  .col-sm-10
+    .issuable-form-select-holder
+      = form.select(:target_branch, issuable.target_branches, { include_blank: true }, { class: 'target_branch select2 span2', disabled: issuable.new_record?, data: { placeholder: "Select branch" }})
+    - if issuable.new_record?
+       
+      = link_to 'Change branches', mr_change_branches_path(issuable)
+
+- if issuable.can_remove_source_branch?(current_user)
+  .form-group
+    .col-sm-10.col-sm-offset-2
+      .checkbox
+        = label_tag 'merge_request[force_remove_source_branch]' do
+          = hidden_field_tag 'merge_request[force_remove_source_branch]', '0', id: nil
+          = check_box_tag 'merge_request[force_remove_source_branch]', '1', issuable.force_remove_source_branch?
+          Remove source branch when merge request is accepted.