credential: use git_prompt instead of git_getpass

credential: use git_prompt instead of git_getpass

We use git_getpass to retrieve the username and password
from the terminal. However, git_getpass will not echo the
username as the user types. We can fix this by using the
more generic git_prompt, which underlies git_getpass but
lets us specify an "echo" option.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>