clippy: fix mut_mut lint

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

Closes #12321

Cargo.toml

@@ -196,6 +196,7 @@ len_without_is_empty = "allow"    # we're not a library crate
 let_and_return = "allow"
 manual_range_contains = "allow"
 map_unwrap_or = "warn"
+mut_mut = "warn"
 needless_lifetimes = "allow"
 new_without_default = "allow"
 option_map_unit_fn = "allow"

src/complete.rs

@@ -2305,7 +2305,7 @@ pub fn complete_add(
 
     // Lock the lock that allows us to edit the completion entry list.
     let mut completion_map = COMPLETION_MAP.lock().expect("mutex poisoned");
-    let c = &mut completion_map
+    let c = completion_map
         .entry(CompletionEntryIndex {
             name: cmd,
             is_path: cmd_is_path,