fallback SSH_ASKPASS when GIT_ASKPASS not set

If GIT_ASKPASS is not set and SSH_ASKPASS set, GIT_ASKPASS will
use SSH_ASKPASS.

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -54,6 +54,9 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 {
 	int handled = 0;
 
+	if (!getenv("GIT_ASKPASS") && getenv("SSH_ASKPASS"))
+		setenv("GIT_ASKPASS", getenv("SSH_ASKPASS"), 1);
+
 	while (*argc > 0) {
 		const char *cmd = (*argv)[0];
 		if (cmd[0] != '-')