libgit2 for commits WORK-IN-PROGRESS

- 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.

Signed-off-by: Sven Strickroth <email@cs-ware.de>