credential: do not mask the username

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

The `credential_ask_one()` function internally uses `git_prompt()` which
in case it is given the flag PROMPT_ASKPASS uses masked input method
instead of git_terminal_prompt, which does not mask user input.

This fixes https://github.com/git-for-windows/git/issue/675

Signed-off-by: Jarosław Honkis <yaras6@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

credential.c

@@ -136,7 +136,9 @@ 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);