tests/test-debugindexdot.t

context: drop support for looking up context by ambiguous changeid (API)

This removes support for using the changectx constructor (and thereby
repo[x]) for looking up contexts by a stringified int, a namespace key
(e.g. a bookmark), or a partial hex nodeid. This means that
e.g. repo[<hex nodeid>] will now fail even if a bookmark with the same
name exists (which is a good thing IMO). It also means that doing
repo[<non-existent node>] no longer ends up loading namespaces (which
was a surprising side-effect of creating of failing to create a
context object that I recently ran into while debugging something
unrelated to this series).

Differential Revision: https://phab.mercurial-scm.org/D3449

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 a
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ cd ..