Auto merge of #6463 - xFrednet:5234-shared-code-in-if-blocks, r=phansch

New Lint: `branches_sharing_code`

This lint checks if all `if`-blocks contain some statements that are the same and can be moved out of the blocks to prevent code duplication. Here is an example:

```rust
let _ = if ... {
    println!("Start"); // <-- Lint for code duplication
    let _a = 99;
    println!("End"); // <-- Lint for code duplication
    false
} else {
    println!("Start");
    let _b = 17;
    println!("End");
    false
};
```

This could be written as:

```rust
println!("Start");

let _ = if ... {
    let _a = 99;
    false
} else {
    let _b = 17;
    false
};

println!("End");
```

---

This lint will get masked by the `IF_SAME_THEN_ELSE` lint. I think it makes more sense to only emit one lint per if block. This means that the folloing example:

```rust
if ... {
    let _a = 17;
} else {
    let _a = 17;
}
```

Will only trigger the `IF_SAME_THEN_ELSE` lint and not the `SHARED_CODE_IN_IF_BLOCKS` lint.

---

closes: #5234

changelog: Added a new lint: `branches_sharing_code`

And hello to the one that is writing the changelog for this release :D

CHANGELOG.md

@@ -2129,6 +2129,7 @@ Released 2018-09-13
 [`borrowed_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrowed_box
 [`box_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_vec
 [`boxed_local`]: https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local
+[`branches_sharing_code`]: https://rust-lang.github.io/rust-clippy/master/index.html#branches_sharing_code
 [`builtin_type_shadow`]: https://rust-lang.github.io/rust-clippy/master/index.html#builtin_type_shadow
 [`bytes_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#bytes_nth
 [`cargo_common_metadata`]: https://rust-lang.github.io/rust-clippy/master/index.html#cargo_common_metadata