run_external_diff: clean up error handling

When the external diff reports an error, we try to clean up and die. However, we can make this process a bit simpler: 1. We do not need to bother freeing memory, since we are about to exit. Nor do we need to clean up our tempfiles, since the atexit() handler will do it for us. So we can die as soon as we see the error. 3. We can just call die() rather than fprintf/exit. This does technically change our exit code, but the exit code of "1" is not meaningful here. In fact, it is probably wrong, since "1" from diff usually means "completed successfully, but there were differences". And while we're there, we can mark the error message for translation, and drop the full stop at the end to make it more like our other messages. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>