builtin run_command: do not exit with -1.

There are shells that do not correctly detect an exit code of -1 as a
failure. We simply truncate the status code to the lower 8 bits.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -256,7 +256,7 @@ static int run_command(struct cmd_struct *p, int argc, const char **argv)
 
 	status = p->fn(argc, argv, prefix);
 	if (status)
-		return status;
+		return status & 0xff;
 
 	/* Somebody closed stdout? */
 	if (fstat(fileno(stdout), &st))