libgit2 for commits WORK-IN-PROGRESS

Tasks todo:
- check for uselibgit2 is missing
- not all errors reported to user
- not all objects are correctly freed

There is a "problem" in libgit2, gitattributes is searched for every git_index_addbypath call.

@ch3cooli @lznuaa Can you please help testing and add help reviewing?

This should fix http://code.google.com/p/tortoisegit/issues/detail?id=1085