tests/test-debugindexdot.t

histedit: renaming parts to which _histedit was split

I recently broke _histedit into multiple functions, each of which
had 'action' in its name. This is a little bit confusing since 'action'
is the word for an individual histedit plan item (such as edit or pick).
To avoid this confusion, these functions are now renamed to contain
'histedit' as part of their names.

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 ..