Make "cd src_dir; git stash" possible

bin/performance_regression.py

@@ -30,7 +30,7 @@ def regression_summary():
     data1 = parse_data()
 
     # Switch to reference data
-    subprocess.call(sys.argv[1:])
+    subprocess.call(sys.argv[1:], shell=True)
 
     # Get reference timings
     run()