CI: Not showing stdout nor stderr when checkout ref

tools/ci/checkout_project_ref.py

@@ -68,7 +68,7 @@ if __name__ == "__main__":
 
     for candidate in candidate_branches:
         try:
-            subprocess.check_call(["git", "checkout", candidate])
+            subprocess.check_call(["git", "checkout", candidate], stdout=subprocess.PIPE, stderr=subprocess.PIPE)  # not print the stdout nor stderr
             print("CI using ref {} for project {}".format(candidate, args.project))
             break
         except subprocess.CalledProcessError: