tests/test-debugindexdot.t

merge: delay debug messages for merge actions

Show messages at a point where the actions have been sorted, thus preparing for
backout of 760c0d67ce5e.

This makes manifestmerge more of a silent operation, just like 'copies' is.

Indent 'preserving' messages to make them subordinate to the action logging so
they fit in the new context. (The 'preserving' messages are quite redundant and
could also be removed completely.)

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 ..