Allow core.askpass to override SSH_ASKPASS.

Modify handling of the 'core.askpass' option so that it has the same effect as
GIT_ASKPASS also if SSH_ASKPASS is set.

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

connect.c

@@ -629,7 +629,8 @@ char *git_getpass(const char *prompt)
 	askpass = getenv("GIT_ASKPASS");
 	if (!askpass)
 		askpass = askpass_program;
-
+	if (!askpass)
+		askpass = getenv("SSH_ASKPASS");
 	if (!askpass || !(*askpass))
 		return getpass(prompt);