Implement the git-receive-pack process in Java

Implement the git-receive-pack process in Java

This implementation provides a basic git-receive-pack service within
Java.  Two hooks APIs are supported before and after commands are
executed within the connection, allowing daemons to customize the
behavior of the updates.

Logic to bind the ReceivePack class to a pipe or network socket is
omitted, as it depends on the transport.  SSH servers will need a
pure Java SSH implementation such as Apache MINA SSHD.  Anonymous
push over git:// requires a basic git-daemon functionality.  Local
pipe access might use pure-Java pipes, or System.in/System.out.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>