Implement std::fmt::Display for FullNameRef, PartialName, and PartialNameRef

[Copilot]

This PR implements the missing std::fmt::Display trait for the reference name types in gix-ref, completing the Display implementation coverage for all reference name types.

Changes

• FullNameRef: Added Display implementation to gix-ref/src/fullname.rs
• PartialName: Added Display implementation to gix-ref/src/name.rs
• PartialNameRef: Added Display implementation to gix-ref/src/name.rs

All implementations follow the same pattern as the existing FullName Display implementation by delegating to the underlying BString/BStr Display implementation.

Usage

With these changes, users can now format all reference name types consistently:

use gix_ref::{FullName, PartialName};
use std::convert::TryFrom;

// FullName (already worked)
let full_name = FullName::try_from("refs/heads/main").unwrap();
println!("{}", full_name); // "refs/heads/main"

// FullNameRef (now works)
let full_name_ref = full_name.as_ref();
println!("{}", full_name_ref); // "refs/heads/main"

// PartialName (now works)  
let partial_name = PartialName::try_from("heads/main").unwrap();
println!("{}", partial_name); // "heads/main"

// PartialNameRef (now works)
let partial_name_ref = partial_name.as_ref();
println!("{}", partial_name_ref); // "heads/main"

Testing

Added comprehensive tests covering all four reference name types with various reference patterns (branches, remotes, tags, HEAD, etc.) to ensure the Display implementations work correctly with both format!() and .to_string().

All existing tests continue to pass, confirming no regressions were introduced.

Original prompt

Implement std::fmt::Display for gix_ref::FullName and gix_ref::FullNameRef. Do the same for PartialName and PartialNameRef for completeness.

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