Stop MR conflict code from blowing up when branches are missing

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/32544

I'll create a separate MR for %9.2 since some refactoring happened in the mean time.

app/models/merge_request.rb

@@ -293,6 +293,8 @@ class MergeRequest < ActiveRecord::Base
   attr_writer :target_branch_sha, :source_branch_sha
   def source_branch_head
+    return unless source_project
+
     source_branch_ref = @source_branch_sha || source_branch
     source_project.repository.commit(source_branch_ref) if source_branch_ref
   end