Add a new option 'core.askpass'.

Setting this option has the same effect as setting the environment variable
'GIT_ASKPASS'.

Signed-off-by: Knut Franke <k.franke@science-computing.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

connect.c

@@ -621,12 +621,14 @@ int finish_connect(struct child_process *conn)
 
 char *git_getpass(const char *prompt)
 {
-	char *askpass;
+	const char *askpass;
 	struct child_process pass;
 	const char *args[3];
 	static struct strbuf buffer = STRBUF_INIT;
 
 	askpass = getenv("GIT_ASKPASS");
+	if (!askpass)
+		askpass = askpass_program;
 
 	if (!askpass || !(*askpass))
 		return getpass(prompt);