New option 'commitstyle' currently supports:

   default - default style, includes commig log and summary of changed files
   diff    - include a diff, like 'git show' does by default
   wdiff    - like diff, but word-diff (useful for LaTeX, etc.)

Also a new option 'showcommitargs' that allows you to replace the args to
'git commit' completely. Supercedes the old 'showcommitextra' option,
which we weren't using on any of our repos.