sync: assign manifest_name earlier

* manifest_name was never set if opt.smart_sync or opt.smart_tag is used.
* Set it earlier, so that the code handles it correctly when it is None.
* An UnboundLocalError is raised if running `repo sync` without any options:
  local variable 'manifest_name' referenced before assignment
* This fixes the above regression caused by commit
  53a6c5d93a8ba708208826dab64c55fe97f06d0b

Change-Id: I57086670f3589beea8461ce0344f6ec47ab85b7b

subcmds/sync.py

@@ -487,6 +487,8 @@ later is required to fix a server side protocol bug.
     if opt.manifest_name:
       self.manifest.Override(opt.manifest_name)
 
+    manifest_name = opt.manifest_name
+
     if opt.smart_sync or opt.smart_tag:
       if not self.manifest.manifest_server:
         print('error: cannot smart sync: no manifest server defined in '
@@ -526,7 +528,6 @@ later is required to fix a server side protocol bug.
                                                     (username, password),
                                                     1)
 
-      manifest_name = opt.manifest_name
       try:
         server = xmlrpc.client.Server(manifest_server)
         if opt.smart_sync: