config: don't read the same config file twice

In some cases some config files would be read twice and shown twice in
showconfig --debug.

mercurial/scmposix.py

@@ -21,6 +21,7 def systemrcpath():
     # old mod_python does not set sys.argv
     if len(getattr(sys, 'argv', [])) > 0:
         p = os.path.dirname(os.path.dirname(sys.argv[0]))
+        if p != '/':
         path.extend(_rcfiles(os.path.join(p, root)))
     path.extend(_rcfiles('/' + root))
     return path

mercurial/scmwindows.py

@@ -40,7 +40,7 def userrcpath():
     path = [os.path.join(home, 'mercurial.ini'),
             os.path.join(home, '.hgrc')]
     userprofile = os.environ.get('USERPROFILE')
-    if userprofile:
+    if userprofile and userprofile != home:
         path.append(os.path.join(userprofile, 'mercurial.ini'))
         path.append(os.path.join(userprofile, '.hgrc'))
     return path