Restore accidentally-removed line.

Commit 88e66d19 is to blame.

Masahiko Sawada

Discussion: http://postgr.es/m/CAD21AoAXeb7O4hgg+efs8JT_SxpR4doAH5c5s-Z5WoRLstBZJA@mail.gmail.com

src/backend/access/transam/subtrans.c

@@ -10,6 +10,7 @@
  * The tree can easily be walked from child to parent, but not in the
  * opposite direction.
  *
+ * This code is based on xact.c, but the robustness requirements
  * are completely different from pg_xact, because we only need to remember
  * pg_subtrans information for currently-open transactions.  Thus, there is
  * no need to preserve data over a crash and restart.