tests/test-dirstate-race.t

dirstate: add a C implementation for nonnormalentries

Before this patch, there was only a python version of nonnormalentries.
On mozilla-central we have a 10x win by putting this function in C:
% python -m timeit -s \
        'from mercurial import hg, ui, parsers; \
        repo = hg.repository(ui.ui(), "mozilla-central"); \
        m = repo.dirstate._map' \
        'parsers.nonnormalentries(m)'

100 loops, best of 3: 3.15 msec per loop

The python implementation runs in 31ms, a similar test gives:
10 loops, best of 3: 31.7 msec per loop

On our big repos, the win is still of 10x with the python implementation running
in 350ms and the C implementation running in 30ms.

  $ hg init
  $ echo a > a
  $ hg add a
  $ hg commit -m test

Do we ever miss a sub-second change?:

  $ for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do
  >     hg co -qC 0
  >     echo b > a
  >     hg st
  > done
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a
  M a