Fix indent for merge_imports

[A4-Tacks]

Example

mod tests {
    use foo$0::bar;
    use foo::baz;
    fn feature() {}
}

Before this PR

mod tests {
    use foo::{bar, baz};
        fn feature() {}
}

After this PR

mod tests {
    use foo::{bar, baz};
    fn feature() {}
}