test_i18ngrep: hard deprecate and forbid its use

Since v2.44.0-rc0~109 (Merge branch 'sp/test-i18ngrep', 2023-12-27)
none of the tests we have, either in 'master' or in flight and
collected in 'seen', use test_i18ngrep.

Perhaps it is good time to update test_i18ngrep to BUG to avoid
people adding new calls to it.

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

t/test-lib-functions.sh

@@ -1263,9 +1263,8 @@ test_cmp_bin () {
 	cmp "$@"
 }
 
-# Deprecated - do not use this in new code
 test_i18ngrep () {
-	test_grep "$@"
+	BUG "do not use test_i18ngrep---use test_grep instead"
 }
 
 test_grep () {