We need `sha` reference from `diff_base_commit` to generate the diff

app/models/merge_request.rb

@@ -332,14 +332,14 @@ class MergeRequest < ActiveRecord::Base
   #
   # see "git diff"
   def to_diff
-    target_project.repository.diff_text(diff_base_commit, source_sha)
+    target_project.repository.diff_text(diff_base_commit.sha, source_sha)
   end
 
   # Returns the commit as a series of email patches.
   #
   # see "git format-patch"
   def to_patch
-    target_project.repository.format_patch(diff_base_commit, source_sha)
+    target_project.repository.format_patch(diff_base_commit.sha, source_sha)
   end
 
   def hook_attrs