bind j instead of M-g to magit-jump-to-diffstats

Do so in the two relevant keymaps `magit-commit-mode-map' and
`magit-diff-mode-map'.  In other maps `j' is bound to the related
`magit-section-jump-map'.
magit.el: explicitly set coding system to utf-8

Unlike the other libraries `magit.el' contains some non-ascii
characters.  Usually Emacs used utf-8 if nothing else is specified
but for some language environments this does not seem to be the case.