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
  53a6c5d9

Change-Id: I57086670f3589beea8461ce0344f6ec47ab85b7b