"git status" can spend a lot of cycles to compute the relation
between the current branch and its upstream, which can now be
disabled with "--no-ahead-behind" option.

* jh/status-no-ahead-behind:
  status: support --no-ahead-behind in long format
  status: update short status to respect --no-ahead-behind
  status: add --[no-]ahead-behind to status and commit for V2 format.
  stat_tracking_info: return +1 when branches not equal