How can I view git diff for any commit using vim-fugitive?

vim-fugitive side-by-side git diff is great for viewing diff of unstaged files.

How can I use vim-fugitive to git diff

• staged files?
• any git revision?

Diff between current file and the index

:Gdiff :0

Diff between current file and some other [revision]

:Gdiff [revision]

Diff between current file and current file 3 commits ago:

:Gdiff ~3