magit-wash-log: use author/date overlay only with oneline style

Do not display the author/date overlay for log style `long', only
for `oneline'.  When the style is `long' then displaying the overlay
fails *and* it is unnecessary because the author information is
already displayed in another fashion.
magit-wash-log: give the code some room to breath
magit-wash-log: remove an unnecessary let-bound variable

Instead of binding USE-AUTHOR-DATE to the value of

  (eq style 'oneline)

just perform that test twice.  That is cheap enough; it might be even
cheaper than a variable binding.  Regardless, I think it improves
readability a lot.