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.
It's gone in libgit2, so we'd have to fake this in libgit2sharp to do that, or revert the change in libgit2 to keep it obsolete for a while.