Function delete_ref_dropping_history 

pub fn delete_ref_dropping_history(
    layout: &RepoLayout,
    branch: &str,
) -> Result<(), RefError>

mkit branch -m helper: deletes the OLD name’s ref after a rename and, on --features history-mmr builds, also destroys its history-MMR journal partition (issue #648).

Unlike delete_ref_recording_history, this does NOT refuse the checked-out branch — branch -m legitimately renames the current branch and moves HEAD to the new name immediately after this call. The NEW name’s ref is created first by the caller (via write_ref_recording_history, which seeds it with a fresh journal), so by the time this runs the old and new incarnations are already disjoint; this just makes sure the OLD name’s journal is not left behind to be inherited by a future branch of the same name.

• Default build (no history-mmr) — exactly refs::delete_ref(layout, branch).
• --features history-mmr — routes through mkit_core::refs::delete_ref_with_history.