Buffer overflow output stream

Most callers/users of TemporaryBuffer are sizing the in-memory
portion large enough that most outputs fit into RAM. With this
assumption they don't pay close attention to the size of IOs
being written, as it "should" just be a copy from one byte array
to another.

Overflow sets up a local file handle, which is costly to write to
for small IO units. Wrap the local file in a BufferedOutputStream
to combine small writes together. Larger writes can still bypass the
buffer as BOS automatically avoids copying for larger writes.

Change-Id: I09f4136dd65c48830cfda86d9101bc647581018a

org.eclipse.jgit/src/org/eclipse/jgit/util/TemporaryBuffer.java

@@ -44,6 +44,7 @@
 
 package org.eclipse.jgit.util;
 
+import java.io.BufferedOutputStream;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
@@ -435,7 +436,7 @@ public abstract class TemporaryBuffer extends OutputStream {
 
 		protected OutputStream overflow() throws IOException {
 			onDiskFile = File.createTempFile("jgit_", ".buf", directory); //$NON-NLS-1$ //$NON-NLS-2$
-			return new FileOutputStream(onDiskFile);
+			return new BufferedOutputStream(new FileOutputStream(onDiskFile));
 		}
 
 		public long length() {