prompt: use git_terminal_prompt

Our custom implementation of git_terminal_prompt has many
advantages over regular getpass(), as described in the prior
commit.

This also lets us implement a PROMPT_ECHO flag for callers
who want it.

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

prompt.h

@@ -2,6 +2,7 @@
 #define PROMPT_H
 
 #define PROMPT_ASKPASS (1<<0)
+#define PROMPT_ECHO    (1<<1)
 
 char *git_prompt(const char *prompt, int flags);
 char *git_getpass(const char *prompt);