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 <yaras6@gmail.com>

credential.c

@@ -135,7 +135,9 @@ static void credential_getpass(struct credential *c)
 {
 	if (!c->username)
 		c->username = credential_ask_one("Username", c,
-						 PROMPT_ASKPASS|PROMPT_ECHO);
+						 (getenv("GIT_ASKPASS") ?
+						  PROMPT_ASKPASS : 0) |
+						 PROMPT_ECHO);
 	if (!c->password)
 		c->password = credential_ask_one("Password", c,
 						 PROMPT_ASKPASS);