debugindex: add --format flag to allow debugging parentdelta -f 1 shows extra data from revlogv1 (aka revlogng) including flags and size (based on a patch by Pradeepkumar Gayam <in3xes@gmail.com>)

Ancestors of 5
4 2 0 
Ancestors of 6 and 5
3 4 2 1 0 
Ancestors of 5 and 4
4 2 0 

Descendants of 5
7 8 
Descendants of 5 and 3
6 7 8 
Descendants of 5 and 4
5 7 8