bisect: replace hardcoded "bad|good" by variables

bisect: replace hardcoded "bad|good" by variables

To add new tags like old/new and have keywords less confusing, the
first step is to avoid hardcoding the keywords.

The default mode is still bad/good.

Signed-off-by: Antoine Delaite <redacted>
Signed-off-by: Louis Stuber <redacted>
Signed-off-by: Valentin Duperray <redacted>
Signed-off-by: Franck Jonas <redacted>
Signed-off-by: Lucien Kong <redacted>
Signed-off-by: Thomas Nguy <redacted>
Signed-off-by: Huynh Khoi Nguyen Nguyen <redacted>
Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>