agent: Fix regression with --newsymkey in loopback mode.

* agent/command.c (cmd_get_passphrase): Never repeat in loopback mode;
same as with !OPT_NEWSYMKEY.

In loopback mode there shall not be any repeat because the caller is
expected to do any confirmation before passing a new passphrase to
gpg.

• Fixes-commit: eace4bbe1ded8b01f9ad52ebc1871f2fd13c3a08
• GnuPG-bug-id: T4991
• Signed-off-by: Werner Koch <wk@gnupg.org>