Update libgit2 binaries to 6950dca

https://github.com/libgit2/libgit2/compare/ad00376...6950dca
Remove orphan hack for Commits.QueryBy()
Restore support to filter since current orphan branch
Teach BranchUpdater to set merge branch directly
Stop sorting collections
Add SkippableTheoryAttribute
Add PathCase and related Repository members
Respect core.ignorecase in BuildRelativePathFrom()

Fixes #214
Add PathspecsOptions to Repository.Diff.Compare()

All the overloads can now report and/or fail upon unmatched pathspecs.
By default, the behaviour is strict (exception thrown if no PathspecsOptions
is passed).
Add PathspecsOptions to Repository.Reset()
Add PathspecsOptions to Repository.Index.Unstage()/Stage()