Add git_prompt_behind and git_prompt_exists

This adds two new theme functions for git:
* `git_prompt_behind` works in a identical fashion to `git_prompt_ahead`
  and will output a format variable (`ZSH_THEME_GIT_PROMPT_BEHIND`) if
  the branch is behind.
* `git_prompt_remote` will output one format variable if the branch
  exists on remote (`ZSH_THEME_GIT_PROMPT_REMOTE_EXISTS`), and another
  if the branch is unpublished (`ZSH_THEME_GIT_PROMPT_REMOTE_MISSING`).

The old `git_prompt_ahead` has been changed. Using git log is subject
to formatting in .gitconfig, which can be overridden and will break
this function. Relying on rev-list is much more stable.