The `git-ref-format`[0] and `radicle-git-ext`[1] crates are now
published to crates.io.
Switch the dependency to rely on them. This also required to update
the `git2` dependency to `0.15.0` as well as using the
`vendored-libgit2` feature -- avoiding the openssl dependency too.
[0]: https://crates.io/crates/git-ref-format
[1]: https://crates.io/crates/radicle-git-ext
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>