AnnotatedCommit::refname panicks on option unwrapping

Hello all,

I'm in the process of trying to use git2 to create a worktree pointing to the merge-base of two references.

I figured out how to get the merge-base, but this gives me an Oid, and AFAIU I need a Reference to be able to create a worktree pointing to it, because WorktreeAddOptions requires it. So I'm trying to get a Reference back from this Oid that merge_base returned me.

The way I'm currently trying this is:

    let base_oid = repo
        .merge_base(
            base_ref
                .target()
                .context("getting target for base reference")?,
            to_check_ref
                .target()
                .context("getting target for to-check reference")?,
        )
        .context("finding the merge-base of the base branch and the to-check reference")?;
    let base_annotated_commit = repo
        .find_annotated_commit(base_oid)
        .context("creating an annotated commit for the merge-base")?;
    let base_refname = base_annotated_commit
        .refname()
        .context("retrieving refname from annotated commit for the merge-base")?;
    let base_ref = repo
        .resolve_reference_from_short_name(base_refname)
        .context("retrieving reference for the merge-base")?
        .resolve()
        .context("resolving merge-base reference")?;

However, this panics at the base_annotated_commit.refname() step, with the following backtrace:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/ekleog/.cargo/registry/src/github.com-1ecc6299db9ec823/git2-0.13.17/src/merge.rs:40:84
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: core::option::Option<T>::unwrap
             at /build/rustc-1.49.0-src/library/core/src/option.rs:386:21
   4: git2::merge::AnnotatedCommit::refname_bytes
             at /home/ekleog/.cargo/registry/src/github.com-1ecc6299db9ec823/git2-0.13.17/src/merge.rs:40:18
   5: git2::merge::AnnotatedCommit::refname
             at /home/ekleog/.cargo/registry/src/github.com-1ecc6299db9ec823/git2-0.13.17/src/merge.rs:35:24
   6: nixpkgs_check::run
             at ./src/main.rs:80:24
   7: nixpkgs_check::main
             at ./src/main.rs:142:11
   8: core::ops::function::FnOnce::call_once
             at /build/rustc-1.49.0-src/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

The full reproducer is at https://github.com/Ekleog/nixpkgs-check/tree/libgit2-repro ; it can be reproduced by doing eg. cargo run -- HEAD main, that will get the merge-base between HEAD and main, and then try to check it out in a worktree created by tempdir, without success.

What do you think about this issue? Also, do you know of a way to do what I'm trying to do?

Anyway, thank you for git2, it looks quite neat to use from my documentation-side look at it, even though for now my attempt was cut short :)

Start in src/merge.rs at AnnotatedCommit::refname and refname_bytes, then run the linked reproducer with cargo run -- HEAD main. Check why the merge-base path produces no refname and ensure the reproducer no longer panics while preserving the expected reference or error behavior.