tests/test-backwards-remove.t

rust-parsers: use in-place mutation instead of allocating a new `Vec`

This is not done for the `dirstate-tree` feature, since it lacks `iter_mut`.

Differential Revision: https://phab.mercurial-scm.org/D9136

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls -A
  .hg
  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
  .hg
  a