refactor git_getpass into generic prompt function

This will allow callers to specify more options (e.g.,
leaving echo on). The original git_getpass becomes a slim
wrapper around the new function.

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

prompt.h

@@ -1,6 +1,9 @@
 #ifndef PROMPT_H
 #define PROMPT_H
 
+#define PROMPT_ASKPASS (1<<0)
+
+char *git_prompt(const char *prompt, int flags);
 char *git_getpass(const char *prompt);
 
 #endif /* PROMPT_H */