Add `pS` command for force pushing branches
Add a buffer type for `rebase-amend`.

`rebase-amend` is the mode the git repo is in while editing an old
commit using `git rebase --interactive`. Currently `git rebase
--interactive` must be called from outside Vim(starting the interactive
rebase from Vim is the big feature I plan for v1.4.0).
Add the `rn` keymap for renaming branches
Bump minor version