use singular for "diffstat"

Rename the section type `diffstats' to `diffstat',
rename `magit-jump-to-diffstats' to `magit-jump-to-diffstat', and
rename `magit-diff-wash-diffstats' to `magit-diff-wash-diffstats'.
magit-jump-to-diffstat-or-diff: new command

replacing `magit-jump-to-diffstat'.