You can use Mathf.Atan2 to compute the angle between 2D vectors. This only works for 2d rotations though, however often that's all you need even in a 3d game. Eg:

var localTarget = transform.InverseTransformPoint(target.position);
var targetAngle = Mathf.Atan2(localTarget.x, localTarget.z);

However, the "angle" will be in radians, not degrees, so you most probably want to convert it to degrees before using it, like this:

var targetAngle = Mathf.Atan2(localTarget.x, localTarget.z) * Mathf.Rad2Deg;

This should give you the target angle in the range -180 to 180 degrees.

You can test to see if the angle returned it more than 180 and then reverse the rotation your self (-360).

Something like:

function getAngle(){
  // angle code
  if (Angle > 180) Angle -= 360;
  return Angle;
}

// do your rotation

A nice trick for 3d: the cross product of two vectors is a vector quantity with a magnitude equal to the product of the magnitudes of the two vectors times the sine of the angle between them.

So for two normalised vectors:

angle_diff = asin(cross(v1, v2).len())

The resulting vector can be used as an axis for rotation between the two input vectors.