Fix merge conflict reading for new diffs

app/models/merge_request.rb

@@ -204,7 +204,7 @@ class MergeRequest < ActiveRecord::Base
 
   def diff_start_commit
     if persisted?
-      merge_request_diff.start_commit
+      merge_request_diff.start_commit || target_branch_head
     else
       target_branch_head
     end
@@ -212,7 +212,7 @@ class MergeRequest < ActiveRecord::Base
 
   def diff_head_commit
     if persisted?
-      merge_request_diff.head_commit
+      merge_request_diff.head_commit || source_branch_head
     else
       source_branch_head
     end

lib/gitlab/conflict/file_collection.rb

@@ -5,7 +5,7 @@ module Gitlab
 
       def initialize(merge_request)
         @merge_request = merge_request
-        @our_commit = merge_request.diff_head_commit.raw.raw_commit
+        @our_commit = merge_request.source_branch_head.raw.raw_commit
         @their_commit = merge_request.target_branch_head.raw.raw_commit
       end