Fix clippy::mut_mut (pedantic)

“this expression mutably borrows a mutable reference. Consider
reborrowing”

https://rust-lang.github.io/rust-clippy/master/index.html#mut_mut

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/isort/mod.rs

@@ -204,7 +204,7 @@ fn normalize_imports(imports: Vec<AnnotatedImport>) -> ImportBlock {
                             entry.inline.push(comment.value);
                         }
                     } else {
-                        let entry = &mut block
+                        let entry = block
                             .import_from_as
                             .entry((
                                 ImportFromData { module, level },