vim-patch:9.1.0447: completion may be wrong when deleting all chars (#29040)

Problem:  completion may be wrong when deleting all chars.
Solution: reset compl_shown_match (glepnir).

closes: https://github.com/vim/vim/pull/14854

53387c55a1

src/nvim/insexpand.c

@@ -1198,6 +1198,10 @@ static int ins_compl_build_pum(void)
   // match after it, don't highlight anything.
   bool shown_match_ok = match_at_original_text(compl_shown_match);
 
+  if (strequal(compl_leader, compl_orig_text) && !shown_match_ok) {
+    compl_shown_match = compl_no_select ? compl_first_match : compl_first_match->cp_next;
+  }
+
   compl_T *shown_compl = NULL;
   bool did_find_shown_match = false;
   int cur = -1;

test/old/testdir/test_ins_complete.vim

@@ -700,14 +700,14 @@ func Test_pum_with_preview_win()
   CheckScreendump
 
   let lines =<< trim END
-      funct Omni_test(findstart, base)
-	if a:findstart
-	  return col(".") - 1
-	endif
-	return [#{word: "one", info: "1info"}, #{word: "two", info: "2info"}, #{word: "three", info: "3info"}]
-      endfunc
-      set omnifunc=Omni_test
-      set completeopt+=longest
+    funct Omni_test(findstart, base)
+      if a:findstart
+        return col(".") - 1
+      endif
+      return [#{word: "one", info: "1info"}, #{word: "two", info: "2info"}, #{word: "three", info: "3info"}]
+    endfunc
+    set omnifunc=Omni_test
+    set completeopt+=longest
   END
 
   call writefile(lines, 'Xpreviewscript')

test/old/testdir/test_popup.vim

@@ -1178,6 +1178,8 @@ func Test_CompleteChanged()
   set completeopt=menu,menuone
   call feedkeys("i\<C-X>\<C-O>\<BS>\<BS>\<BS>f", 'tx')
   call assert_equal('five', g:word)
+  call feedkeys("i\<C-X>\<C-O>\<BS>\<BS>\<BS>f\<BS>", 'tx')
+  call assert_equal('one', g:word)
 
   autocmd! AAAAA_Group
   set complete& completeopt&