status: add --[no-]ahead-behind to status and commit for V2 format.

status: add --[no-]ahead-behind to status and commit for V2 format.

Teach "git status" and "git commit" to accept "--no-ahead-behind"
and "--ahead-behind" arguments to request quick or full ahead/behind
reporting.

When "--no-ahead-behind" is given, the existing porcelain V2 line
"branch.ab +x -y" is replaced with a new "branch.ab +? -?" line.
This indicates that the branch and its upstream are or are not equal
without the expense of computing the full ahead/behind values.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>