Always allocate the PackOutputStream copyBuffer

Always allocate the PackOutputStream copyBuffer

The getCopyBuffer() is almost always used during output. All known
implementations of ObjectReuseAsIs rely on the buffer to be present,
and the only sane way to get good performance from PackWriter is to
reuse objects during packing.

Avoid a branch and test when obtaining this buffer by making sure
it is always populated.

Change-Id: I200baa0bde5dcdd11bab7787291ad64535c9f7fb