Change if_same_then_else to be a style lint

CC #3770

From https://github.com/rust-lang/rust-clippy/issues/3770#issuecomment-687565594 (@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 @@
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub IF_SAME_THEN_ELSE,
-    correctness,
+    style,
     "`if` with the same `then` and `else` blocks"
 }
 

tests/ui/needless_bool_assign.stderr

@@ -48,7 +48,8 @@ LL |       } else {
 LL | |         a.field = true;
 LL | |     }
    | |_____^
-   = note: `#[deny(clippy::if_same_then_else)]` on by default
+   = note: `-D clippy::if-same-then-else` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::if_same_then_else)]`
 
 error: aborting due to 4 previous errors