Use `diff_base_commit` instead of `target_branch` to generate diffs

app/controllers/projects/merge_requests_controller.rb

@@ -57,8 +57,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     respond_to do |format|
       format.html
       format.json { render json: @merge_request }
-      format.diff { render text: @merge_request.to_diff(current_user) }
-      format.patch { render text: @merge_request.to_patch(current_user) }
+      format.diff { render text: @merge_request.to_diff }
+      format.patch { render text: @merge_request.to_patch }
     end
   end
 

app/models/merge_request.rb

@@ -331,15 +331,15 @@ class MergeRequest < ActiveRecord::Base
   # Returns the raw diff for this merge request
   #
   # see "git diff"
-  def to_diff(current_user)
-    target_project.repository.diff_text(target_branch, source_sha)
+  def to_diff
+    target_project.repository.diff_text(diff_base_commit, source_sha)
   end
 
   # Returns the commit as a series of email patches.
   #
   # see "git format-patch"
-  def to_patch(current_user)
-    target_project.repository.format_patch(target_branch, source_sha)
+  def to_patch
+    target_project.repository.format_patch(diff_base_commit, source_sha)
   end
 
   def hook_attrs