diff --whitespace=warn/error: obey blank-at-eof

The "diff --check" code used to conflate trailing-space whitespace error
class with this, but now we have a proper separate error class, we should
check it under blank-at-eof, not trailing-space.

The whitespace error is not about _having_ blank lines at end, but about
adding _new_ blank lines.  To keep the message consistent with what is
given by "git apply", call whitespace_error_string() to generate it,
instead of using a hardcoded custom message.

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

diff.c

@@ -1650,10 +1650,14 @@ static void builtin_checkdiff(const char *name_a, const char *name_b,
 		ecb.priv = &data;
 		xdi_diff(&mf1, &mf2, &xpp, &xecfg, &ecb);
 
-		if ((data.ws_rule & WS_TRAILING_SPACE) &&
+		if ((data.ws_rule & WS_BLANK_AT_EOF) &&
 		    data.trailing_blanks_start) {
-			fprintf(o->file, "%s:%d: ends with blank lines.\n",
-				data.filename, data.trailing_blanks_start);
+			static char *err;
+
+			if (!err)
+				err = whitespace_error_string(WS_BLANK_AT_EOF);
+			fprintf(o->file, "%s:%d: %s\n",
+				data.filename, data.trailing_blanks_start, err);
 			data.status = 1; /* report errors */
 		}
 	}

t/t4015-diff-whitespace.sh

@@ -335,10 +335,10 @@ test_expect_success 'line numbers in --check output are correct' '
 
 '
 
-test_expect_success 'checkdiff detects trailing blank lines' '
+test_expect_success 'checkdiff detects new trailing blank lines (1)' '
 	echo "foo();" >x &&
 	echo "" >>x &&
-	git diff --check | grep "ends with blank"
+	git diff --check | grep "new blank line"
 '
 
 test_expect_success 'checkdiff allows new blank lines' '

t/t4019-diff-wserror.sh

@@ -165,7 +165,7 @@ test_expect_success 'trailing empty lines (1)' '
 
 	rm -f .gitattributes &&
 	test_must_fail git diff --check >output &&
-	grep "ends with blank lines." output &&
+	grep "new blank line at" output &&
 	grep "trailing whitespace" output
 
 '