resolves #193: simpler commit tables

@kevgo 

Can now remove the duplication from all commit tables. I removed them from just git-extract for now. Will remove it from the others, but wanted to give you a chance to look at it now.
This is a much better approach, thanks for cleaning this up!

FYI monkey-patching is global and therefore runs into the same issues that monkey-patching has in JavaScript, i.e. collisions of two monkey-patching libraries. Ruby therefore introduces refinements (http://www.ruby-doc.org/core-2.1.5/doc/syntax/refinements_rdoc.html). I'm not sure whether they are available and stable at this point, hence this is just an FYI. Once Ruby 2.2 comes out this Xmas, it will hopefully contain them.