dirstate-tree: move a conditional in an explicit boolean

This will help readability a bit and make the next change simpler to read.

Differential Revision: https://phab.mercurial-scm.org/D9202

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

@@ -57,9 +57,11 b' impl Node {'
         let head = split.next().unwrap_or(b"");
         let tail = split.next().unwrap_or(b"");
 
+        // Are we're modifying the current file ? Is the the end of the path ?
+        let is_current_file = tail.is_empty() && head.is_empty();
+
         if let NodeKind::File(file) = &mut self.kind {
-            if tail.is_empty() && head.is_empty() {
-                // We're modifying the current file
+            if is_current_file {
                 let new = Self {
                     kind: NodeKind::File(File {
                         entry: new_entry,