stat_tracking_info: return +1 when branches not equal

stat_tracking_info: return +1 when branches not equal

Extend stat_tracking_info() to return +1 when branches are not equal and to
take a new "enum ahead_behind_flags" argument to allow skipping the (possibly
expensive) ahead/behind computation.

This will be used in the next commit to allow "git status" to avoid full
ahead/behind calculations for performance reasons.

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