tests/test-dirstate-race.t

changelog: lazily parse description

Before, the description field was converted to a localstr at parse
time. With this patch, we store the raw description and convert to
a localstr when it is first accessed.

We see a revset speedup for revsets that don't access the description:

author(mpm)
0.896565
0.914234
0.869085

date(2015)
0.878797
0.891980
0.862525

extra(rebase_source)
0.865446
0.912514
0.871500

author(mpm) or author(greg)
1.801832
1.860402
1.791589

date(2015) or branch(default)
0.968276
0.994673
0.974027

author(mpm) or desc(bug) or date(2015) or extra(rebase_source)
3.656193
3.721032
3.643593

As you can see, most of these revsets are already faster than from
before this refactoring: we have already offset the performance
loss from the introduction of the new class representing parsed
changelog entries!

  $ 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