tests/test-diff-copy-depth.t

manifest: make one use of _mancache avoid manifestctxs

In a future patch we will change manifestctx and treemanifestctx to no longer
derive from manifestdict and treemanifest, respectively. This means that
consumers of the _mancache will now need to be aware of the different between
the two, until we get rid of the manifest entirely and the _mancache becomes
only filled with ctxs.

This fixes one case of it that can be fixed by using the other cache. Future
patches will address the others uses using the upcoming manifestctx.read()
function.

  $ for i in aaa zzz; do
  >     hg init t
  >     cd t
  > 
  >     echo
  >     echo "-- With $i"
  > 
  >     touch file
  >     hg add file
  >     hg ci -m "Add"
  > 
  >     hg cp file $i
  >     hg ci -m "a -> $i"
  > 
  >     hg cp $i other-file
  >     echo "different" >> $i
  >     hg ci -m "$i -> other-file"
  > 
  >     hg cp other-file somename
  > 
  >     echo "Status":
  >     hg st -C
  >     echo
  >     echo "Diff:"
  >     hg diff -g
  > 
  >     cd ..
  >     rm -rf t
  > done
  
  -- With aaa
  Status:
  A somename
    other-file
  
  Diff:
  diff --git a/other-file b/somename
  copy from other-file
  copy to somename
  
  -- With zzz
  Status:
  A somename
    other-file
  
  Diff:
  diff --git a/other-file b/somename
  copy from other-file
  copy to somename