status: warn when a/b calculation takes too long

status: warn when a/b calculation takes too long

The ahead/behind calculation in 'git status' can be slow in some
cases. Users may not realize that there are ways to avoid this
computation, especially if they are not using the information.

Add a warning that appears if this calculation takes more than
two seconds. The warning can be disabled through the new config
setting advice.statusAheadBehind.

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>