rust: fix formatting

rust: fix formatting

Maybe there is a small divergence in the nightly formatter? This didn't seem to
trip the CI.

Differential Revision: https://phab.mercurial-scm.org/D9131

--- a/rust/hg-core/src/dirstate/dirstate_tree/node.rs	Wed Sep 23 10:02:16 2020 +0200
+++ b/rust/hg-core/src/dirstate/dirstate_tree/node.rs	Wed Sep 30 18:23:43 2020 +0200
@@ -91,9 +91,7 @@
 
         match &mut self.kind {
             NodeKind::Directory(directory) => {
-                Node::insert_in_directory(
-                    directory, new_entry, head, tail,
-                )
+                Node::insert_in_directory(directory, new_entry, head, tail)
             }
             NodeKind::File(_) => {
                 unreachable!("The file case has already been handled")