diff-files: avoid negative exit value

diff-files: avoid negative exit value

If loading the index fails, we print an error and then return "-1" from
the function. But since this is a builtin, we end up with exit(-1),
which produces odd results since program exit codes are unsigned.
Because of integer conversion, it usually becomes 255, which is at least
still an error, but values above 128 are usually interpreted as signal
death.

Since we know the program is exiting immediately, we can just replace
the error return with a die().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>