iris-bot: fetch only master branch

iris-bot

@@ -117,7 +117,7 @@ def time():
     subprocess.run(["git", "fetch", "-q", "https://gitlab.mpi-sws.org/{}".format(iris_repo), iris_rev], check=True)
     test_commit = subprocess.run(["git", "rev-parse", "FETCH_HEAD"], check=True, stdout=subprocess.PIPE).stdout.decode().strip()
     # Determine the base commit in master
-    subprocess.run(["git", "fetch", "-q", "origin"], check=True)
+    subprocess.run(["git", "fetch", "-q", "origin", "master"], check=True)
     base_commit = subprocess.run(["git", "merge-base", test_commit, "origin/master"], check=True, stdout=subprocess.PIPE).stdout.decode().strip()
     # Trigger the builds
     print("Triggering timing builds for {} base commit ({}) and test commit ({}) with ad-hoc ID {}...".format(project_name, base_commit[:8], test_commit[:8], id))