diff: reject unknown change class given to --diff-filter

We used to accept "git diff --diff-filter=Q" (note that there is no
such change class 'Q') silently and showed no output (because there
is no such change class 'Q').

Error out when such an input is given.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3537,7 +3537,7 @@ static int parse_diff_filter_opt(const char *optarg, struct diff_options *opt)
 
 		bit = (0 <= optch && optch <= 'Z') ? filter_bit[optch] : 0;
 		if (!bit)
-			continue; /* ignore unknown ones, like we always have */
+			return optarg[i];
 		opt->filter |= bit;
 	}
 	return 0;