Require NuGet  2.7+ to install the package
Move higher level Index operations to IRepository

Fix #822
Drop -DTHREADSAFE in update scripts
Update libgit2 to 3f8d005
Update git_remote_rename to reflect new libgit2 signature
Remove Remote.IsSupportedUrl

Users should now check the library features to determine if HTTPS
or SSH support exists in the library.
GitMergeHeadHandle -> GitAnnotatedCommitHandle

To reflect the git_merge_head -> git_annotated_commit changes in
libgit2.
GitDiffOptions: ushort -> uint
Update git_remote_rename to reflect libgit2