graft: do not use .remove on a smart set (regression)

Revset calls use to return a list. Graft use to mutate that list. We cannot do
this anymore leading to a crash when grafting multiple changeset with a revset.

File ".../mercurial/commands.py", line 3117, in graft
  revs.remove(rev)
AttributeError: '_addset' object has no attribute 'remove'

We are late in code-freeze so we make the shortest possible fix by turning it
back to a list.