Update libgit2 binaries to 98eaf39

This covers the [change in signature](https://github.com/libgit2/libgit2/compare/e87d9d3...98eaf39#diff-c03573006ee579612b0d55bbb5dba97dL248) of `git_remote_ls`, and includes some fixes to `git_blame_buffer` that are necessary for #552.