tests/test-debugindexdot.t

check-concurrency: expose the feature as 'concurrent-push-mode'

We move the feature to a proper configuration and document it. The config goes
in the 'server' section because it feels like something the server owner would
want to decide. We pick and open field because it seems likely that other
checking levels will emerge in the future. (eg: server like the mozilla-try
server will likely wants a "none" value)

The option name contains 'push' since this affects 'push' only. The option value
'check-related' is preferred over one explicitly containing 'allow' or 'deny'
because the client still have a strong decision power here. Here, the server is
just advising the client on the check mode to use.

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 ..