path: remove unnecessary string copies

As the length of `path` is known at this point, there is no point in
making an exact copy using `slice`.

PR-URL: nodejs#14438
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>

lib/path.js

@@ -767,7 +767,9 @@ const win32 = {
         }
       }
     } else if (code === 47/*/*/ || code === 92/*\*/) {
-      return path[0];
+      // `path` contains just a path separator, exit early to avoid
+      // unnecessary work
+      return path;
     }
 
     for (var i = len - 1; i >= offset; --i) {
@@ -1031,23 +1033,23 @@ const win32 = {
               if (len === 3) {
                 // `path` contains just a drive root, exit early to avoid
                 // unnecessary work
-                ret.root = ret.dir = path.slice(0, 3);
+                ret.root = ret.dir = path;
                 return ret;
               }
               rootEnd = 3;
             }
           } else {
             // `path` contains just a drive root, exit early to avoid
             // unnecessary work
-            ret.root = ret.dir = path.slice(0, 2);
+            ret.root = ret.dir = path;
             return ret;
           }
         }
       }
     } else if (code === 47/*/*/ || code === 92/*\*/) {
       // `path` contains just a path separator, exit early to avoid
       // unnecessary work
-      ret.root = ret.dir = path[0];
+      ret.root = ret.dir = path;
       return ret;
     }