Update libgit2 to 96fb6a6

https://github.com/libgit2/libgit2/compare/98eaf39...96fb6a6
Initial rename detection work
Revise API for rename/copy detection
Remove trailing whitespaces
Introduce ObjectDatabase.CalculateHistoryDivergence()

Fix #562
Explicitly dispose handle
Merge pull request #1 from libgit2/vNext

Pull upstream