git-p4: clone does not use --git-dir

Complain if --git-dir is given during a clone.  It has no
effect.  Only --destination and --bare can change where the newly
cloned git dir will be.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -2335,7 +2335,8 @@ def main():
     args = sys.argv[2:]
 
     if len(options) > 0:
-        options.append(optparse.make_option("--git-dir", dest="gitdir"))
+        if cmd.needsGit:
+            options.append(optparse.make_option("--git-dir", dest="gitdir"))
 
         parser = optparse.OptionParser(cmd.usage.replace("%prog", "%prog " + cmdName),
                                        options,