tests/test-backwards-remove.t

upgrade: walk the source store file only once

I don't expect this to have a significant performance impact, but it seems
simpler and saner to do the operation only once and to keep the result around.

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a