Update libgit2 to 0a62918

Updating libgit2 to `0a62918`.  The current version in HEAD (`0383fa`) had an unfortunate bug updating the tree cache (https://github.com/libgit2/libgit2/issues/2629) that can cause some issues.
How about `string.Format("Remote '{0}' hasn't been renamed as it doesn't exist.", name)`?