Use M-d over M-g for magit-jump-to-diffstats

M-g is bound by default to the prefix goto-map, which is still useful to
call for navigation when inside of a magit buffer. M-d is by default
kill-word. I believe magit-mode-map is only applied in read only
circumstances, so replacing M-d does not clobber useful bindings.

See #1448 for discussion.