tests/test-debugindexdot.t

subrepo: pull revisions on demand when archiving hg subrepos

Before, running 'hg archive -S' could result in

 abort: unknown revision '65903cebad86f1a84bd4f1134f62fa7dcb7a1c98'!

if a subrepo was missing completely or had missing changesets. Now,
the missing changesets will be pulled or cloned as appropriate.

This make Mercurial subrepos match Git subrepos which already took
care to fetch any missing commits before starting the archive.

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
  }