magit-show-diffstat: fix doc-string typo
set :package-version for custom options

For the appropriate custom options set the `:package-version' keyword,
using "2.0.0" as the version.  Replace some occurrences of the "1.3.0"
version; there never was such a release, and that won't change.

Always make `:package-version' the first keyword.
consistently order keywords in `defcustom's

The order is
`:package-version'
`:prefix'
`:set-after'
`:set'
`:type'.