tests/test-dirstate-race.t

largefiles: handle that a found standin file doesn't exist when removing it

I somehow ended up in a situation where hg crashed on an unlink I introduced in
328545c7d8a1.

I don't know how it happened and can't reproduce it. It seems like it only can
happen when the file is removed between the time of check in a working
directory context walk that finds a standin file, and the time of use when we
try to remove it because the corresponding largefile doesn't exist.

But better safe than sorry: replace the plain unlink with unlinkpath with
ignoremissing=True. That will also remove remaining empty directories, which
arguably is more correct.

  $ 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