bug#10181: split `diff-refine-change' in several faces

[Dani Moncayo]

In this bug report, I requested to replace the face
'diff-refine-change' with three different faces, related to the faces
'diff-added', 'diff-removed' and 'diff-changed'.

That was implemented time ago (thanks Juri!), but I've just realized a
little inconsistency in the name of one of the new faces.  The new
faces were:
* 'diff-refine-added', related to 'diff-added'.
* 'diff-refine-removed', related to 'diff-removed'.
* 'diff-refine-change', related to 'diff-changed'.

So, I think that 'diff-refine-change' should be named
'diff-refine-changed' instead, for consistency.

-- 
Dani Moncayo