Add Repository.Stashes.Add
Add Repository.Stashes.GetEnumerator()
Add implicit cast from GitOid(?) to ObjectId

Also, support marshaling NULL in OidSafeHandle
Use implicit GitOid to ObjectId cast everywhere
Give props to Jon Skeet

Add link back to source Stack Overflow post for the CopyStream
method after obtaining permission from Jon Skeet to include in
LibGit2Sharp.
Teach BranchUpdater to set merge branch directly