diff-mode auto-refines only after a successful motion

diff-mode auto-refines only after a successful motion

Prior to this patch (if enabled) auto-refinement would kick in after all
hunk navigation commands, even if the motion failed.  This would result
in a situation where the hunk navigation would signal an error and beep,
but yet still accomplish potentially useful work, by auto-refining.
This patch moves the auto-refinement code to only run when a motion was
successful

* lisp/vc/diff-mode.el (diff--internal-hunk-next,
  diff--internal-hunk-prev): Removed auto-refinement-triggering code
* lisp/vc/diff-mode.el (diff--wrap-navigation): Added
  auto-refinement-triggering code