py3: restore perfstartup() prior to b456b2e0ad9f on Windows

Otherwise the test errors out with:

    --- c:/Users/Matt/projects/hg_py3/tests/test-contrib-perf.t
    +++ c:/Users/Matt/projects/hg_py3/tests/test-contrib-perf.t.err
    @@ -184,6 +184,8 @@
       $ hg perfrevrange
       $ hg perfrevset 'all()'
       $ hg perfstartup
    +  'b'c:' is not recognized as an internal or external command,
    +  operable program or batch file.
       $ hg perfstatus
       $ hg perftags
       $ hg perftemplating

contrib/perf.py

@@ -970,13 +970,13 b' def perfindex(ui, repo, **opts):'
 def perfstartup(ui, repo, **opts):
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
-    cmd = fsencode(sys.argv[0])
     def d():
         if os.name != r'nt':
-            os.system(b"HGRCPATH= %s version -q > /dev/null" % cmd)
+            os.system(b"HGRCPATH= %s version -q > /dev/null" %
+                      fsencode(sys.argv[0]))
         else:
             os.environ[r'HGRCPATH'] = r' '
-            os.system(r"%s version -q > NUL" % cmd)
+            os.system(r"%s version -q > NUL" % sys.argv[0])
     timer(d)
     fm.end()