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