dirstate-tree: simplify the control flow in the Node.insert method

But explicitly with the special case early, laying out the various case become
simpler.

(The initial motivation was to make some future lifetime error simpler).

Differential Revision: https://phab.mercurial-scm.org/D9203

rust/hg-core/src/dirstate/dirstate_tree/node.rs

@@ -60,7 +60,23 b' impl Node {'
         // Are we're modifying the current file ? Is the the end of the path ?
         let is_current_file = tail.is_empty() && head.is_empty();
 
+        // Potentially Replace the current file with a directory if it's marked
+        // as `Removed`
+        if !is_current_file {
         if let NodeKind::File(file) = &mut self.kind {
+                if file.entry.state == EntryState::Removed {
+                    self.kind = NodeKind::Directory(Directory {
+                        was_file: Some(Box::from(file.clone())),
+                        children: Default::default(),
+                    })
+                }
+            }
+        }
+        match &mut self.kind {
+            NodeKind::Directory(directory) => {
+                Node::insert_in_directory(directory, new_entry, head, tail)
+            }
+            NodeKind::File(file) => {
             if is_current_file {
                 let new = Self {
                     kind: NodeKind::File(File {
@@ -68,36 +84,23 b' impl Node {'
                         ..file.clone()
                     }),
                 };
-                return InsertResult {
+                    InsertResult {
                     did_insert: false,
                     old_entry: Some(std::mem::replace(self, new)),
-                };
+                    }
             } else {
                 match file.entry.state {
-                    // Only replace the current file with a directory if it's
-                    // marked as `Removed`
                     EntryState::Removed => {
-                        self.kind = NodeKind::Directory(Directory {
-                            was_file: Some(Box::from(file.clone())),
-                            children: Default::default(),
-                        })
+                            unreachable!("Removed file turning into a directory was dealt with earlier")
                     }
                     _ => {
-                        return Node::insert_in_file(
+                            Node::insert_in_file(
                             file, new_entry, head, tail,
                         )
                     }
                 }
             }
         }
-
-        match &mut self.kind {
-            NodeKind::Directory(directory) => {
-                Node::insert_in_directory(directory, new_entry, head, tail)
-            }
-            NodeKind::File(_) => {
-                unreachable!("The file case has already been handled")
-            }
         }
     }