tests/test-debugindexdot.t

test-rebase: add a brute force test

Rebase is becoming more complex and it looks like a good idea to try some
brute force enumeration to cover cases that are hard to find manually.

Using brute force to generate repos in different shapes and enumerating the
rebase source and destination would generate too many cases that takes too
long to compute. This patch limits the "brute force" to only the "rebase
source" part. Repo and destination are still manual.

The added test cases are crafted manually to reveal some behaviors that are
not covered by other tests:

  - "revlog index out of range" crash
  - after rebase, p1 == p2, p2 != null
  - "nothing to merge" abort

In the future we might want to add more tests here. For now I'm more
interested in revealing interesting behaviors in a minified way. I tried
some more complex cases but didn't find other interesting behaviors.

Differential Revision: https://phab.mercurial-scm.org/D262

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 .hg/store/data/a.i
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ cd ..