perf: add a configurable sleep on startup

This is intended to counteract power management by giving a consistent
idle period before test runs.

contrib/perf.py

@@ -14,6 +14,10 b' def gettimer(ui, opts=None):'
 
     This functions exist to gather the creation of formatter in a single
     place instead of duplicating it in all performance command."""
+
+    # enforce an idle period before execution to counteract power management
+    time.sleep(ui.configint("perf", "presleep", 1))
+
     if opts is None:
         opts = {}
     # redirect all to stderr