Add gitmail support for a few different display styles
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.
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