`magit-wash-log': call `derived-mode-p' only once

Doing the same thing twice for no additional benefit is always a pain,
so `let'-bind the return value of `derived-mode-p' around the body form.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #903 from praet/wash-log

`magit-wash-log': call `derived-mode-p' only once