Checkout HEAD instead of master

In cargo, `HEAD` is updated instead of `master`. This mismatch means
that the two refs can drift, depending on when cargo and this library
last fetched changes.

I believe this change will fix crate-ci/cargo-release#224.

https://github.com/rust-lang/cargo/blob/0b2059e9811caa238a603f0300d9c5fb485000a6/src/cargo/sources/registry/remote.rs#L56

src/lib.rs

@@ -312,7 +312,7 @@ impl Index {
         let mut origin_remote = repo
             .find_remote("origin")
             .or_else(|_| repo.remote_anonymous(INDEX_GIT_URL))?;
-        origin_remote.fetch(&["master"], Some(&mut fetch_opts()), None)?;
+        origin_remote.fetch(&["HEAD"], Some(&mut fetch_opts()), None)?;
         let oid = repo.refname_to_id("FETCH_HEAD")?;
         let object = repo.find_object(oid, None).unwrap();
         repo.reset(&object, git2::ResetType::Hard, None)?;