add: show diff between current and base branch

If you are using [git-bash-completion](https://github.com/markgandolfo/git-bash-completion) you may consider change your PS1 to also show differences between the current branch and the base branch: PS1='[\u@\h \W$(__git_ps1_diff_base " %s")$(__git_ps1 " (%s)")]\$ '
  * If your branch is 11 commits ahead and 30 commits behind master bash will show:

          [user@host ▴11 ▾30 branch]$