Add staged-sign (git diff --cached)

Resolve https://github.com/emacsorphanage/git-gutter/issues/129 This PR want to add git staged sign, which is `git diff --cached` <img width="957" height="787" alt="image" src="https://github.com/user-attachments/assets/e71cdc6e-6e2e-4985-b70e-23a94bb446e0" />