Fix rebase onto master

CHANGELOG-EE

 v 8.1.0 (unreleased)
   - added an issues template (Hannes Rosenögger)
+  - Fix "Rebase onto master"
+
 v 8.1.0
   - Add documentation for "Share project with group" API call 
   - Abiliy to disable 'Share with Group' feature (via UI and API)

app/models/merge_request.rb

@@ -514,7 +514,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def source_sha_parent
-    source_project.repository.commit(source_sha).parents.first.sha
+    source_project.repository.commit(commits.last.sha).parents.first.sha
   end
 
   def ff_merge_possible?

app/views/projects/merge_requests/widget/open/_ff_accept.html.haml

@@ -13,7 +13,7 @@
     remote: false, method: :post, html: { class: 'accept-mr-form js-requires-input' } do |f|
     = hidden_field_tag :authenticity_token, form_authenticity_token
     .accept-merge-holder.clearfix.js-toggle-container
-      - if @merge_request.target_project.merge_requests_rebase_enabled && can_rebase?(@merge_request.target_project, @merge_request.target_branch)
+      - if @merge_request.target_project.merge_requests_rebase_enabled && can_rebase?(@merge_request.source_project, @merge_request.source_branch)
         - if @merge_request.rebase_in_progress?
           %h4 Rebase in progress... It can take a while. Reload at will.
         - else

spec/models/merge_request_spec.rb

@@ -211,6 +211,12 @@ describe MergeRequest do
     end
   end
 
+  describe "#source_sha_parent" do
+    it "returns the sha of the parent commit of the first commit in the MR" do
+      expect(subject.source_sha_parent).to eq("ae73cb07c9eeaf35924a10f713b364d32b2dd34f")
+    end
+  end
+
   it_behaves_like 'an editable mentionable' do
     subject { create(:merge_request) }