add a changelog entry for #1124

- add a changelog entry for #1124
- correct duplicate entry for 3.1.12 -> 3.1.13

Author: larsks

doc/source/changes.rst

@@ -2,7 +2,13 @@
 Changelog
 =========
 
-3.1.12
+3.1.??
+======
+
+* git.Commit objects now have a ``replace`` method that will return a
+  copy of the commit with modified attributes.
+
+3.1.13
 ======
 
 See the following for details: