Git: Undo Last Commit…
git.undoLastCommit Take the last commit off the branch, keep its changes staged and put its message back in the commit box (git reset --soft HEAD~1).
How to run it
Open the command palette (M-x in the Emacs keymap) and search for Git: Undo Last Commit…, then press Enter. It has no default keybinding, but you can bind one in Settings → Keymaps.