util: avoid referencing `time.clock()` on Windows when missing (issue6238)

It's been removed in 3.8, and issues a deprecation warning since 3.3.

Differential Revision: https://phab.mercurial-scm.org/D7780

mercurial/util.py

@@ -2057,16 +2057,17 b' def checkwinfilename(path):'
             )
 
 
+timer = getattr(time, "perf_counter", None)
+
 if pycompat.iswindows:
     checkosfilename = checkwinfilename
-    timer = time.clock
+    if not timer:
+        timer = time.clock
 else:
     # mercurial.windows doesn't have platform.checkosfilename
     checkosfilename = platform.checkosfilename  # pytype: disable=module-attr
-    timer = time.time
-
-if safehasattr(time, "perf_counter"):
-    timer = time.perf_counter
+    if not timer:
+        timer = time.time
 
 
 def makelock(info, pathname):