Use libgit2 for commits

There is a small "problem" in libgit2, gitattributes is searched for every git_index_addbypath call (https://github.com/libgit2/libgit2/pull/1291 might fix this issue and also reduces lots of other git_config code).

@ch3cooli @lznuaa Can you please help testing and add help reviewing?

This should fix http://code.google.com/p/tortoisegit/issues/detail?id=1085