Update libgit2 binaries to 2f6f6eb.

https://github.com/libgit2/libgit2/compare/58eea5e...2f6f6eb
Introduce RemoteCollection.Remove()
BranchCollection: Obsolete Move and introduce Rename.
Simplify disposal of some resources
Fix wrong handle lifecycle

git_branch_delete() only frees the handle upon success.
Release LibGit2Sharp v0.18.0
Test -> CanResolveConflictsByRemovingFromTheIndex

We're not clearing the conflicts, we're resolving them.  The
difference is that removing from the index means that the items
get moved to the REUC.  "Clearing" or "removing" (in libgit2
terminology, anyway) means that they are removed without being
resolved (or going into the REUC).
Introduce Index.Conflicts.ResolvedConflicts
Introduce Repository.Index.Conflicts.Names