SF bug # 493951 string.{starts,ends}with vs slices

Handle negative indices similar to slices.
diff --git a/Misc/NEWS b/Misc/NEWS
index 383614f..4a949f4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -6,6 +6,9 @@
 
 Core and builtins
 
+- Fixed string.startswith and string.endswith builtin methods
+  so they accept negative indices.  [SF bug 493951]
+
 - Fixed a bug with a continue inside a try block and a yield in the
   finally clause.  [SF bug 567538]