It's difficult to debug a sporadic problem when automatic diff
refinement doesn't display refined overlays on some diff hunks
occasionally.  Looking at the code, it seems there is a possible
race condition when diff--font-lock-refined calls diff-hunk-next
that sets a timer that later might remove and try to re-add again
overlays already added by the direct call of diff--refine-hunk
from diff--font-lock-refined.  Also diff-hunk-next prevents
multiple consequent calls from refining a set of hunks
by checking diff--auto-refine-data.

To avoid such double refinement, this patch let-binds
diff-auto-refine-mode to nil before calling diff-hunk-next
for non-interactive navigation (it calls diff--refine-hunk
explicitly below):