grep: fix more Turkish-eyes bugs

Fix more bugs recently uncovered by Norihiro Tanaka (Bug#43577).
* NEWS: Mention new bug report.
* src/grep.c (ok_fold): New static var.
(setup_ok_fold): New function.
(fgrep_icase_charlen): Reject single-byte characters
if they match some multibyte characters when ignoring case.
This part of the patch is partly derived from
<https://bugs.gnu.org/43577#14>, which means it is:
Co-authored-by: Norihiro Tanaka <noritnk@kcn.ne.jp>
(main): Call setup_ok_fold if ok_fold might be needed.
* src/searchutils.c (kwsinit): With the grep.c changes,
this code can now revert to classic 7th Edition Unix style;
aborting would be wrong.
* tests/turkish-eyes: Add tests for these bugs.

1 files changed, 15 insertions, 3 deletions

diff --git a/tests/turkish-eyes b/tests/turkish-eyes
index ba1ea33b..879b59d4 100755
--- a/tests/turkish-eyes
+++ b/tests/turkish-eyes
@@ -36,11 +36,23 @@ i=$(printf '\304\261') # lowercase dotless i
 
       data="I:$I $i:i"
 search_str="$i:i I:$I"
-printf "$data\n" > in || framework_failure_
+printf "$data\\n" > in || framework_failure_
 
 for opt in -E -F -G; do
-  LC_ALL=$L grep $opt -i "$search_str" in > out || fail=1
-  compare out in || fail=1
+  for pat in i I "$i" "$I" " " : "$search_str"; do
+    LC_ALL=$L grep $opt -i "$pat" in > out || fail=1
+    compare in out || fail=1
+
+    case $pat in
+      i|"$I") printf "$I\\ni\\n";;
+      I|"$i") printf "I\\n$i\\n";;
+      :) printf ":\\n:\\n";;
+      ' ') printf " \\n";;
+      *) cat in;;
+    esac >exp || framework_failure_
+    LC_ALL=$L grep -o $opt -i "$pat" in > out || fail=1
+    compare exp out || fail=1
+  done
 done
 
 Exit $fail