Handle if/else with empty/comment line

[schneems]

Reported in #187 this code:

class Foo
  def foo
    if cond?
      foo
    else
      # comment
    end
  end

  # ...

  def bar
    if @recv
    end_is_missing_here
  end
end

Triggers an incorrect suggestion:

Unmatched keyword, missing `end' ?
   1  class Foo
   2    def foo
>  3      if cond?
>  5      else
   8    end
  16  end

Part of the issue is that we're using newlines to determine when to stop and pause while scanning. This is useful for determining logically smaller chunks to evaluate, but in this case, it causes us to pause before grabbing the "end" that is right below the newline. This problem is similar to #179.

However, in the case of expanding the same indentation "neighbors," I want to always grab all empty values at the end of the scan. I don't want to do that when changing indentation levels, as it affects scan results.

There may be some way to normalize this behavior between the two, but the tests really don't like that change.

To fix this issue for expanding against different indentations I needed a way to first try and grab any additional newlines with the ability to rollback that guess. In #192 I experimented with decoupling scanning from the AroundBlockScan logic. It also added the ability to take a snapshot of the current scanner and rollback to prior changes.

With this ability in place now we:

• Grab extra empties before looking at the above/below line for the matching keyword/end statement
• If there's a match, grab it
• If there's no match, discard the newlines we picked up in the evaluation

That solves the issue.