Add the EE `rebase_commit_sha` migration back in its original location

db/migrate/20160621123729_add_rebase_commit_sha_to_merge_requests.rb

+# This migration is a duplicate of 20171230123729_add_rebase_commit_sha_to_merge_requests_ce.rb
+#
+# We backported this feature from EE using the same migration, but with a new
+# timestamp, which caused an error when the backport was then to be merged back
+# into EE.
+#
+# See discussion at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3932
+class AddRebaseCommitShaToMergeRequests < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    unless column_exists?(:merge_requests, :rebase_commit_sha)
+      add_column :merge_requests, :rebase_commit_sha, :string
+    end
+  end
+
+  def down
+    if column_exists?(:merge_requests, :rebase_commit_sha)
+      remove_column :merge_requests, :rebase_commit_sha
+    end
+  end
+end