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

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackOutputStream.java

@@ -67,9 +67,9 @@ public final class PackOutputStream extends OutputStream {
 
 	private long count;
 
-	private byte[] headerBuffer = new byte[32];
+	private final byte[] headerBuffer = new byte[32];
 
-	private byte[] copyBuffer;
+	private final byte[] copyBuffer = new byte[16 << 10];
 
 	private long checkCancelAt;
 
@@ -216,8 +216,6 @@ public final class PackOutputStream extends OutputStream {
 
 	/** @return a temporary buffer writers can use to copy data with. */
 	public byte[] getCopyBuffer() {
-		if (copyBuffer == null)
-			copyBuffer = new byte[16 * 1024];
 		return copyBuffer;
 	}