"/history" 'start' default value.

The fakebank got adapted to the same logic from the
Python bank (with negative deltas, 'start' defaults to
UINT64_MAX, whereas with positives it defaults to 0).

src/bank-lib/fakebank.c

@@ -804,8 +804,10 @@ handle_history (struct TALER_FAKEBANK_Handle *h,
     GNUNET_assert (0);
     return MHD_NO;
   }
+
   if (NULL == start)
-    pos = h->transactions_tail;
+    pos = 0 > count ? h->transactions_tail : h->transactions_head;
+
   else if (NULL != h->transactions_head)
   {
     for (pos = h->transactions_head;