vim-patch:9.1.1296: completion: incorrect truncation logic

Problem:  completion: incorrect truncation logic (after: v9.1.1284)
Solution: replace string allocation with direct screen rendering and
          fixe RTL/LTR truncation calculations (glepnir)

closes: vim/vim#17081

d4dbf822dc

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/optionstr.c

@@ -2133,6 +2133,7 @@ static const struct chars_tab fcs_tab[] = {
   CHARSTAB_ENTRY(&fcs_chars.eob,        "eob",       "~", NULL),
   CHARSTAB_ENTRY(&fcs_chars.lastline,   "lastline",  "@", NULL),
   CHARSTAB_ENTRY(&fcs_chars.trunc,      "trunc",     ">", NULL),
+  CHARSTAB_ENTRY(&fcs_chars.truncrl,    "truncrl",   "<", NULL),
 };
 
 static lcs_chars_T lcs_chars;