Fix MR diff_refs

app/models/merge_request.rb

@@ -518,7 +518,7 @@ class MergeRequest < ActiveRecord::Base
     @ci_commit ||= source_project.ci_commit(last_commit.id) if last_commit && source_project
   end
 
-  def diff_range
-    [last_commit.parent, first_commit]
+  def diff_refs
+    [first_commit.parent || first_commit, last_commit]
   end
 end

app/views/projects/merge_requests/_new_submit.html.haml

@@ -38,7 +38,7 @@
       = render "projects/merge_requests/show/commits"
     #diffs.diffs.tab-pane.active
       - if @diffs.present?
-        = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @merge_request.diff_range
+        = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @merge_request.diff_refs
       - elsif @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
         .alert.alert-danger
           %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.

app/views/projects/merge_requests/show/_diffs.html.haml

@@ -1,6 +1,6 @@
 - if @merge_request_diff.collected?
   = render "projects/diffs/diffs", diffs: params[:w] == '1' ? @merge_request.diffs_no_whitespace : @merge_request.diffs,
-    project: @merge_request.project, diff_refs: @merge_request.diff_range
+    project: @merge_request.project, diff_refs: @merge_request.diff_refs
 - elsif @merge_request_diff.empty?
   .nothing-here-block Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
 - else