perf: offer perf.stub to only run one loop

contrib/perf.py

@@ -29,8 +29,15 b' def gettimer(ui, opts=None):'
     ui.fout = ui.ferr
     # get a formatter
     fm = ui.formatter('perf', opts)
+    # stub function, runs code only once instead of in a loop
+    # experimental config: perf.stub
+    if ui.configbool("perf", "stub"):
+        return functools.partial(stub_timer, fm), fm
     return functools.partial(_timer, fm), fm
 
+def stub_timer(fm, func, title=None):
+    func()
+
 def _timer(fm, func, title=None):
     results = []
     begin = time.time()