Update libgit2 to 3f8d005

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.
`[Obsolete]` with a trivial implementation (e.g. `url != null && url.IndexOf(':') > 0`)?