fix bug: magit-log crashes emacs.

I found mini bug, 'magit-log-author-date-overlay' overlays are not deleted.
it can crash emacs.

> 1. Open magit-log buffer. `l l`
> 2. On the magit-log buffer, re-open magit-log. and repeat. `l l, l l, ...`
> 3. Emacs explode...

I changed magit-log-author-date-overlay deleting method,
 use overlay property 'evaporate'. overlays will delete itself.