v1.3.0: Rebasing functionality and more

• Add the rb keymap to start a rebase.
  • The rebase buffer supports the merge buffer's keymaps(R, <Cr>,
    aa, A).
  • The rebase buffer also support ra to abort the rebase, rc to
    continue to the next patch and rs to skip the current patch.
• Add MerginalToggle command to toggle the merginal buffer.
• Add MerginalClose command to close the merginal buffer.
• Support ps, pl and pf directly on remote branches.
• Fix a bug with non-linear merges that don't have merge conflicts.