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>)

http://mercurial.selenic.com/bts/issue433

  $ hg init
  $ echo a > a
  $ hg commit -Ama
  adding a

  $ hg parents -r 0 doesnotexist
  abort: 'doesnotexist' not found in manifest!
  [255]