perf: perform a garbage collection before each iteration

Currently, no explicit garbage collection is performed when running
the microbenchmarks in `hg perf`. I think this is wrong because
garbage collection can have a significant impact on execution times.
And, if gc is triggered via the default heuristics, it will
fire effectively randomly during subsequent benchmark iterations
due to variable amount of garbage left over from previous runs.

Running a gc before invoking the measured function will help ensure
state is more consistent across all iterations.

contrib/perf.py

@@ -20,6 +20,7 b''
 
 from __future__ import absolute_import
 import functools
+import gc
 import os
 import random
 import sys
@@ -189,6 +190,7 b' def stub_timer(fm, func, title=None):'
     func()
 
 def _timer(fm, func, title=None):
+    gc.collect()
     results = []
     begin = util.timer()
     count = 0