Add atime tests and fix atime capture in metadata.from_path().

Add atime tests and fix atime capture in metadata.from_path().

Adjust metadata.from_path() to accept an optional statinfo argument.
This allows us to pass in the value returned by recursive_dirlist()
during a save_tree().  Otherwise the second lstat(), in from_path(),
will receive an incorrect atime, one which has been adjusted by the
recursive_dirlist() stat().  Of course, this also allows us to drop an
unnecessary lstat() call.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>