tests/test-debugindexdot.t

ui: defer setting pager related properties until the pager has spawned

When --pager=on is given, dispatch.py spawns a pager before setting up color.
If the pager failed to launch, ui.pageractive was left set to True, so color
configured itself based on 'color.pagermode'.  A typical MSYS setting would be
'color.mode=auto, color.pagermode=ansi'.  In the failure case, this would print
a warning, disable the pager, and then print the raw ANSI codes to the terminal.

Care needs to be taken, because it appears that leaving ui.pageractive=True was
the only thing that prevented an attempt at running the pager again from inside
the command.  This results in a double warning message, so pager is simply
disabled on failure.

The ui config settings didn't need to be moved to fix this, but it seemed like
the right thing to do for consistency.

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 ..