status: add status.aheadbehind setting

status: add status.aheadbehind setting

The --[no-]ahead-behind option was introduced in fd9b544a
(status: add --[no-]ahead-behind to status and commit for V2
format, 2018-01-09). This is a necessary change of behavior
in repos where the remote tracking branches can move very
quickly ahead of the local branches. However, users need to
remember to provide the command-line argument every time.

Add a new "status.aheadBehind" config setting to change the
default behavior of all git status formats.

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