Add an internal tree copy implementation

Add an internal tree copy implementation

The general idea is to copy everything, preserving access mode, times,
xattrs on files. An attempt to create the new object is first made,
followed by an unlink and a repeated attempt. This optimizes for the
case where the output tree is empty, which seems to be the common
thing in our case.

Reflink is attempted for normal files.

Symlinks are not adjusted: if they pointed outside of the tree being
copied, they might be dangling now.

Block and character device nodes and any other special files are not
copied.