tests/test-backwards-remove.t

tests: suppress "Checked out 1 paths of <hash>" from modern git

test-convert-git.t is failiing since git commit 0f086e6dca (checkout:
print something when checking out paths, 2018-11-13). Suppress the new
output by disambiguating the arguments with a "--" separator.

Differential Revision: https://phab.mercurial-scm.org/D5625

  $ 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