Do not mask the username when reading credentials

When user is asked for credentials there is no need to mask username,
so PROMPT_ASKPASS flag on calling credential_ask_one for login is
unnecessary.

credential_ask_one internally uses git_prompt which in case of given
flag PROMPT_ASKPASS uses masked input method instead of
git_terminal_prompt, which does not mask user input.

This fixes #675

Signed-off-by: yaras <[email protected]>

Author: yaras