perf: ensure HGRCPATH is exported on Windows After dropping the garbage collector hack, `hg perfstartup` started yelling about not being able to import the evolve extension, which I have in my user config. Launching `env` shows that an empty HGRCPATH isn't exported to the environment. Since `env` doesn't quote, I have no idea if the variable is trimmed, but Mercurial doesn't complain when processing it.

--- a/contrib/perf.py	Fri Jun 16 20:35:45 2017 -0400
+++ b/contrib/perf.py	Fri Jun 16 20:49:32 2017 -0400
@@ -661,7 +661,7 @@
         if os.name != 'nt':
             os.system("HGRCPATH= %s version -q > /dev/null" % cmd)
         else:
-            os.environ['HGRCPATH'] = ''
+            os.environ['HGRCPATH'] = ' '
             os.system("%s version -q > NUL" % cmd)
     timer(d)
     fm.end()