tests/test-dirstate-race.t

shelve: avoid writing file that is never read from

The contents of the .files file has not been used since 1d7a36ff2615
(shelve: use rebase instead of merge (issue4068), 2013-10-23), so stop
writing it. Where we currently use the presence of the file as a check
for a valid shelve name, switch to checking for the .patch file.

  $ 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