Just exercize debugindexdot

Create a short file history including a merge.

  $ hg init t

  $ cd t

  $ echo a > a

  $ hg ci -qAm t1 -d '0 0'

  $ echo a >> a

  $ hg ci -m t2 -d '1 0'

  $ hg up -qC 0

  $ echo b >> a

  $ hg ci -m t3 -d '2 0'

  created new head

  $ HGMERGE=true hg merge -q

  $ hg ci -m merge -d '3 0'

  $ hg debugindexdot .hg/store/data/a.i

  digraph G {

  	-1 -> 0

  	0 -> 1

  	0 -> 2

  	2 -> 3

  	1 -> 3

  }