tests/test-merge-subrepos.t

perf: use a formatter for output

We use a `formatter` object in the perf extensions. This allow the use of
formatted output like json. To avoid adding logic to create a formatter and pass
it around to the timer function in every command, we add a `gettimer` function
in charge of returning a `timer` function as simple as before but embedding an
appropriate formatter.

This new `gettimer` function also return the formatter as it needs to be
explicitly closed at the end of the command.

example output:


  $ hg --config ui.formatjson=True perfvolatilesets visible obsolete
  [
   {
    "comb": 0.02,
    "count": 126,
    "sys": 0.0,
    "title": "obsolete",
    "user": 0.02,
    "wall": 0.0199398994446
   },
   {
    "comb": 0.02,
    "count": 117,
    "sys": 0.0,
    "title": "visible",
    "user": 0.02,
    "wall": 0.0250301361084
   }
  ]

  $ hg init

  $ echo a > a
  $ hg ci -qAm 'add a'

  $ hg init subrepo
  $ echo 'subrepo = http://example.net/libfoo' > .hgsub
  $ hg ci -qAm 'added subrepo'

  $ hg up -qC 0
  $ echo ax > a
  $ hg ci -m 'changed a'
  created new head

  $ hg up -qC 1
  $ cd subrepo
  $ echo b > b
  $ hg add b
  $ cd ..

Should fail, since there are added files to subrepo:

  $ hg merge
  abort: uncommitted changes in subrepository 'subrepo'
  [255]