util: introduce timer()

As documented for timeit.default_timer, there are better timers available for
performance measures on some platforms. These timers don't have a set epoch,
and thus are only useful for interval measurements, but have higher
resolution, and thus get you a better measurement overall.

Use the same selection logic as Python's timeit.default_timer. This is a
platform clock on Python 2 and early Python 3, and time.perf_counter on Python
3.3 and later (where time.perf_counter is introduced as the best timer to use).

mercurial/util.py

@@ -1203,8 +1203,13 b' def checkwinfilename(path):'
 
 if pycompat.osname == 'nt':
     checkosfilename = checkwinfilename
+    timer = time.clock
 else:
     checkosfilename = platform.checkosfilename
+    timer = time.time
+
+if safehasattr(time, "perf_counter"):
+    timer = time.perf_counter
 
 def makelock(info, pathname):
     try: