GH-119169: Slightly speed up os.walk(topdown=True) (GH-121431)

GH-119186: Slightly speed up `os.walk(topdown=True)`

When `os.walk()` traverses into subdirectories in top-down mode, call
`os.path.join()` once to add a trailing slash, and use string concatenation
thereafter to generate child paths.

Lib/os.py

@@ -417,14 +417,16 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
             # Yield before sub-directory traversal if going top down
             yield top, dirs, nondirs
             # Traverse into sub-directories
-            for dirname in reversed(dirs):
-                new_path = join(top, dirname)
-                # bpo-23605: os.path.islink() is used instead of caching
-                # entry.is_symlink() result during the loop on os.scandir() because
-                # the caller can replace the directory entry during the "yield"
-                # above.
-                if followlinks or not islink(new_path):
-                    stack.append(new_path)
+            if dirs:
+                prefix = join(top, top[:0])  # Add trailing slash
+                for dirname in reversed(dirs):
+                    new_path = prefix + dirname
+                    # bpo-23605: os.path.islink() is used instead of caching
+                    # entry.is_symlink() result during the loop on os.scandir() because
+                    # the caller can replace the directory entry during the "yield"
+                    # above.
+                    if followlinks or not islink(new_path):
+                        stack.append(new_path)
         else:
             # Yield after sub-directory traversal if going bottom up
             stack.append((top, dirs, nondirs))