ChangeLog: Mention "current" and "since" versions

ChangeLog.manual

@@ -1,4 +1,4 @@
-Changes since CMake 2.8.1
+Changes in CMake 2.8.2-rc1 (since 2.8.1):
 - Build on Tru64 (#10542)
 - Build on mingw-w64
 - Build on old Sun (#10550, #10543)