added a pure-ruby git library and converted the cat_file commands to use it
# Please enter the commit message for your changes.
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# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
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#	new file:   PURE_TODO
#	modified:   lib/grit.rb
#	new file:   lib/grit/git-ruby.rb
#	new file:   lib/grit/git-ruby/internal/loose.rb
#	new file:   lib/grit/git-ruby/internal/mmap.rb
#	new file:   lib/grit/git-ruby/internal/pack.rb
#	new file:   lib/grit/git-ruby/internal/raw_object.rb
#	new file:   lib/grit/git-ruby/object.rb
#	new file:   lib/grit/git-ruby/repository.rb
#	modified:   lib/grit/git.rb
#	new file:   test/fixtures/cat_file_blob_ruby
#	new file:   test/fixtures/cat_file_commit_ruby
#	new file:   test/fixtures/cat_file_tree_ruby
#	new file:   test/test_rubygit.rb
#