tests/test-dirstate-race.t

dirstate: only call lstat once per flags invocation

This makes a big difference to performance in some cases.

hg --time locate 'set:symlink()'

mozilla-central (70,000 files):

  before: 2.92 sec
  after:  2.47

another repo (170,000 files):

  before: 7.87 sec
  after:  6.86

  $ 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