Fix invalid byte sequence errors when using grep method
git 1.7.4.6 required.

The following tests failed with git 1.7.2.1

* grit/test/test_commit.rb:201
* grit/test/test_repo.rb:422

The minimum version passing tests is 1.7.4.6
Merge pull request #23 from babatakao/patch-1

git 1.7.4.6 required.
Merge pull request #22 from jacargentina/fix-utf8

Fix invalid byte sequence errors when using grep method
Adds "quick_diff" feature to Commit class.

Returns "unbaked" Diff instances with only the a_/b_path, new_file, and
deleted_file props set.