Deprecate SafeBufferedOutputStream

Deprecate SafeBufferedOutputStream

Java 8 fixed the silent flush during close issue by
FilterOutputStream (base class of BufferedOutputStream)
using try-with-resources to close the stream, getting a
behavior matching what JGit's SafeBufferedOutputStream
was doing:

  try {
    flush();
  } finally {
    out.close();
  }

With Java 8 as the minimum required version to run JGit
it is no longer necessary to override close() or have
this class. Deprecate the class, and use the JRE's version
of close.

Change-Id: Ic0584c140010278dbe4062df2e71be5df9a797b3