magit-wash-hunk: fix argument of magit-diff-refine-hunk

In magit-wash-hunk, magit-diff-refine-hunk seems to be called
invalid argument. So error occurs when variable magit-diff-refine-hunk
is set not nil.
Merge pull request #1052 from shigi/fix-refine-hunk

magit-wash-hunk: fix argument of magit-diff-refine-hunk