Merge branch 'tf/prompt-preserve-exit-status'

Using the exit status of the last command in the prompt, e.g.
PS1='$(__git_ps1) $? ', did not work well because the helper
function stomped on the exit status.

* tf/prompt-preserve-exit-status:
  git-prompt: preserve value of $? inside shell prompt

diff --cc contrib/completion/git-prompt.sh
Simple merge