Split an amended commit into a narrow follow-up

Use when a correction was amended into an implementation commit but review requires that correction as a dedicated follow-up commit.

Preconditions

• The working tree is clean.
• The original pre-amend commit still exists (find it with git reflog if needed).
• The current tree is the intended final tree.
• git diff <original> HEAD contains only the correction that should become the follow-up.

Procedure