Switch from getSegments() to containsUpLevelReferences() in `Arti…

…factRoot#asDerivedRoot`.

`getSegments()` iterates over the path string twice and also creates garbage by taking substrings and putting them in a list. `containsUpLevelReferences()` takes constant time, as it does not need to look past the 3rd character of the already-normalized path string.

Additionally, clean up `PathFragment#containsUpLevelReferences` so that it does not perform string concatenation.

PiperOrigin-RevId: 357826526

src/main/java/com/google/devtools/build/lib/actions/ArtifactRoot.java

@@ -107,7 +107,7 @@ public static ArtifactRoot asDerivedRoot(
     // Make sure that we are not creating a derived artifact under the execRoot.
     Preconditions.checkArgument(!execPath.isEmpty(), "empty execPath");
     Preconditions.checkArgument(
-        !execPath.getSegments().contains(".."),
+        !execPath.containsUplevelReferences(),
         "execPath: %s contains parent directory reference (..)",
         execPath);
     Preconditions.checkArgument(

src/main/java/com/google/devtools/build/lib/vfs/PathFragment.java

@@ -614,7 +614,8 @@ public PathFragment toRelative() {
    */
   public boolean containsUplevelReferences() {
     // Path is normalized, so any ".." would have to be the first segment.
-    return normalizedPath.equals("..") || normalizedPath.startsWith(".." + SEPARATOR_CHAR);
+    return normalizedPath.startsWith("..")
+        && (normalizedPath.length() == 2 || normalizedPath.charAt(2) == SEPARATOR_CHAR);
   }
 
   /**