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.

contrib/perf.py

@@ -661,7 +661,7 b' def perfstartup(ui, repo, **opts):'
         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()