Orphan changeset and trying to relocate a node on top of itself (issue6097)
https://bz.mercurial-scm.org/show_bug.cgi?id=6097

  $ . $TESTDIR/testlib/common.sh

  $ cat << EOF >> $HGRCPATH
  > [extensions]
  > rebase =
  > evolve =
  > EOF

  $ hg init issue6097
  $ cd issue6097

  $ echo apricot > a
  $ hg ci -qAm apricot

  $ echo banana > b
  $ hg ci -qAm banana

Let's go back to amend 0 and make an orphan out of 1

  $ hg up -q 0
  $ echo coconut > c
  $ hg add -q c
  $ hg ci --amend -m 'apricot and coconut'
  1 new orphan changesets

Now rebase the successor of 0 on top of 1

  $ hg rebase -r . -d 1
  rebasing 2:32acf8fb1b23 "apricot and coconut" (tip)
  1 new orphan changesets

Pruning 1 just to get it out of the way

  $ hg prune -q 1

Note how both the regular DAG and the obsolescence graph are linear, but the
paths from 3 to 0 are different: 3-1-0 and 3-2-0

  $ hg log -G
  @  changeset:   3:2868fe6df617
  |  tag:         tip
  |  parent:      1:e0486f65907d
  |  user:        test
  |  date:        Thu Jan 01 00:00:00 1970 +0000
  |  instability: orphan
  |  summary:     apricot and coconut
  |
  x  changeset:   1:e0486f65907d
  |  user:        test
  |  date:        Thu Jan 01 00:00:00 1970 +0000
  |  obsolete:    pruned using prune
  |  summary:     banana
  |
  x  changeset:   0:692cc7b6212c
     user:        test
     date:        Thu Jan 01 00:00:00 1970 +0000
     obsolete:    rewritten using amend, rebase as 3:2868fe6df617
     summary:     apricot
  

  $ hg obslog
  @  2868fe6df617 (3) apricot and coconut
  |
  x  32acf8fb1b23 (2) apricot and coconut
  |    rewritten(parent, content) as 2868fe6df617 using rebase by test (Thu Jan 01 00:00:00 1970 +0000)
  |
  x  692cc7b6212c (0) apricot
       rewritten(description, content) as 32acf8fb1b23 using amend by test (Thu Jan 01 00:00:00 1970 +0000)
  

FIXME: obviously a traceback is not good

  $ hg evolve -r . 2>&1 | grep '^\S'
  move:[3] apricot and coconut
  atop:[3] apricot and coconut
  ** unknown exception encountered, please report by visiting
  ** https://mercurial-scm.org/wiki/BugTracker
  ** Python * (glob)
  ** Mercurial Distributed SCM (*) (glob)
  ** Extensions loaded: rebase, evolve
  ** ProgrammingError: tried to relocate a node on top of itself
  ** (This shouldn't happen. If you still need to move changesets, please do so manually with nothing to rebase - working directory parent is also destination)
  Traceback (most recent call last):
  mercurial.error.ProgrammingError: tried to relocate a node on top of itself