clone: Use DirCacheCheckout This simple change lets us get rid of WorkDirCheckout from JGit, and all of its supporting code. Change-Id: I1a5aabe9ab4a2b156fd37cc7e9ededb4ed70f53a Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>