Change if_same_then_else to be a style lint

CC rust-lang#3770

From rust-lang#3770 (comment) (@flip1995):

> Oh I thought I replied to this: I definitely see now that having this
> as a correctness lint might be the wrong categorization. What we might
> want to do is to just allow this lint, if there are comments in the
> arm bodies. But a good first step would be to downgrade this lint to
> style or complexity. I would vote for style since merging two arms is
> not always less complex.

clippy_lints/src/copies.rs

@@ -117,7 +117,7 @@ declare_clippy_lint! {
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub IF_SAME_THEN_ELSE,
-    correctness,
+    style,
     "`if` with the same `then` and `else` blocks"
 }