credential: fix wrong double password prompt

We shouldn't use ssh-askpass to ask for the user, that causes most tools
to ask for "password: username" followed by "password: password".
Totally confusing for the user.

Instead, use a normal prompt to ask for the user.

Reported-by: Chris  Warrick <kwpolska@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>