Make class ChangeSet thread-safe

Class org.apache.maven.scm.ChangeSet declares a static member of type SimpleDateFormat which is not thread-safe.
If 2 concurrent treads try to use it, it will lead to error.

Proposed solution is to use class ThreadSafeDateFormat described here: http://asolntsev.blogspot.com/2009/05/threadsafedateformat.html