Idempotency bug for comments in attribute parameters

Input:
```nix
{ 
  foo
# bar
, baz # qux
}:
null
```

After one formatting:
```nix
{
  foo
  # bar
  ,
  baz, # qux
}:
null
```

After another:
```nix
{
  foo,
  # bar
  baz, # qux
}:
null
```

Note that #217 is needed to not have infinite recursion when using `--verify` with this.

Ping @piegamesde 