diff-highlight settings break whitespace error color in some cases

[patrikndv]

When using the diff-highlight settings, the color.diff.whitespace color is sometimes overridden because the trailing whitespace error becomes colored by highlight instead. This seems to happen at least when the block of change is a single line that is changed to introduce whitespace error. In other words, something like:

-this is a changed line
+this is a changed line(WHITESPACE)

This will color the trailing whitespace in highlight instead of color.diff.whitespace.

[scottchiefbaker]

Are you able to provide a diff file the exhibits this behavior?