Update libgit2 binaries to 11f8336

https://github.com/libgit2/libgit2/compare/32e4992...11f8336
Clarified documentation of BranchTrackingDetails

The properties AheadBy, BehindBy and CommonAncestor are not only null if there is no tracking branch; they are also null if the tracking branch is empty (e.g. after cloning an empty repository)