tests/test-debugindexdot.t

discovery: avoid wrong detection of multiple branch heads (issue6256)

This fix the code using obsolescence markers to remove "to be obsoleted" heads
during the detection of new head creation from push. The code turned out to not
use the branch information at all. This lead changeset from different branch to
be detected as new head on unrelated branch.

The code fix is actually quite simple. New tests have been added to covers
these cases.

Differential Revision: https://phab.mercurial-scm.org/D8259

Just exercise 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 -c
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ hg debugindexdot -m
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ hg debugindexdot a
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ cd ..