chore(fs): slightly reduce memory usage of IsParent (#10223)

### Purpose

Small optimizations for IsParent and IsInternal, to avoid needless
allocations.

lib/fs/util.go

@@ -169,10 +169,16 @@ func IsParent(path, parent string) bool {
 		// not be caught below.
 		return path != "/"
 	}
-	if parent[len(parent)-1] != PathSeparator {
-		parent += pathSeparatorString
+	if parent[len(parent)-1] == PathSeparator {
+		return strings.HasPrefix(path, parent)
 	}
-	return strings.HasPrefix(path, parent)
+	if !strings.HasPrefix(path, parent) {
+		return false
+	}
+	if len(path) <= len(parent) {
+		return false
+	}
+	return path[len(parent)] == PathSeparator
 }
 
 func CommonPrefix(first, second string) string {