Prevent creating folder if input and output file paths are the same

[jfmengels]

I'm looking into ways to make elm-optimize-level-2 faster and this looked like an easy win, though it will likely not change a lot. This will likely be how I run elm-optimize-level-2 in elm-review (it is in a branch), and I imagine others will as well.

[jfmengels]

I updated the check to prevent creating the folder when the folder for the input and output are the same. The diff is now much smaller. In the best case, it adds a call to path.dirname but removes a call to fs.existsSync which would be a lot slower. In the worst case, it would only add a call to path.dirname.

[mdgriffith]

Ah interesting. I didn't realize existsSync was slow, but I guess that makes sense.