revsetbenchmark: fix error raising

We want to display the commands, not all arguments of the function. (The old
code actually crash, failing to joining a list of lists.)

contrib/revsetbenchmarks.py

@@ -22,7 +22,7 b' def check_output(*args, **kwargs):'
     proc = Popen(*args, **kwargs)
     output, error = proc.communicate()
     if proc.returncode != 0:
-        raise CalledProcessError(proc.returncode, ' '.join(args))
+        raise CalledProcessError(proc.returncode, ' '.join(args[0]))
     return output
 
 def update(rev):