tests/test-dirstate-race.t

tests: fix test-update-branches to remove non-conflicting file

I was clearly very sloppy when I wrote the test case for
experimental.updatecheck=noconflict. The test case that checks that
one can move between commits with a removed file was deleting a file
that was modified between the source and target commits, which
resulted in a "change/delete" conflict. Since that is a conlict, the
update correctly failed. Let's fix the test by removing a file that is
not modified between the two commits.

  $ 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