Merge branch 'jh/status-aheadbehind'

"git status" can be told a non-standard default value for the
"--[no-]ahead-behind" option with a new configuration variable
status.aheadBehind.

* jh/status-aheadbehind:
  status: ignore status.aheadbehind in porcelain formats
  status: warn when a/b calculation takes too long
  status: add status.aheadbehind setting

diff --cc Documentation/config/advice.txt
Simple merge

diff --cc advice.c
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc wt-status.c
Simple merge