revsetbenchmark: handle exception case

If the revset being benchmarked has an exception, the handling code was
encountering an error because the exception did not always have an "output"
attribute (I think it's a python 2.7 thing).

contrib/revsetbenchmarks.py

@@ -63,7 +63,7 b' def perf(revset, target=None, contexts=F'
         return parseoutput(output)
     except CalledProcessError as exc:
         print >> sys.stderr, 'abort: cannot run revset benchmark: %s' % exc.cmd
-        if exc.output is None:
+        if getattr(exc, 'output', None) is None: # no output before 2.7
             print >> sys.stderr, '(no output)'
         else:
             print >> sys.stderr, exc.output