Changing some git alias and remappings in vimrc

- git rb: git rebase
- git rbu: Script to rebase upon upstream
- git pr: git pull --rebase
- vim: Rolling back splitted window navigation to use 'Control' instead of
  'Alt'
- vim: Using 'Shift-PageUp' and 'Shift-PageDown' to move tabs and native
  'Control-PageUp' and 'Control-PageDown' to switch between them