tests/test-merge-revert.t

rust: don't use a reference to a `Cow`

This was caught by `clippy`.

  $ hg init repo
  $ cd repo

  $ echo "added file1" > file1
  $ echo "added file2" > file2
  $ hg add file1 file2
  $ hg commit -m "added file1 and file2"

  $ echo "changed file1" >> file1
  $ hg commit -m "changed file1"

  $ hg -q log
  1:08a16e8e4408
  0:d29c767a4b52
  $ hg id
  08a16e8e4408 tip

  $ hg update -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg id
  d29c767a4b52
  $ echo "changed file1" >> file1
  $ hg id
  d29c767a4b52+

  $ hg revert --all
  reverting file1
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  d29c767a4b52

  $ hg update
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  08a16e8e4408 tip

  $ hg update -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo "changed file1" >> file1

  $ hg update
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  08a16e8e4408 tip

  $ hg revert --all
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  08a16e8e4408 tip

  $ hg revert -r tip --all
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  08a16e8e4408 tip

  $ hg update -C
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  08a16e8e4408 tip