vim-patch:8.2.3934: repeating line comment is undesired for "O" command

Problem:    Repeating line comment is undesired for "O" command.
Solution:   Do not copy line comment leader for "O". (closes vim/vim#9426)
5ea5f37372

src/nvim/change.c

@@ -1201,13 +1201,14 @@ int open_line(int dir, int flags, int second_line_indent, bool *did_do_comment)
   // Find out if the current line starts with a comment leader.
   // This may then be inserted in front of the new line.
   end_comment_pending = NUL;
-  if (flags & OPENLINE_DO_COM) {
+  if (flags & OPENLINE_DO_COM && dir == FORWARD) {
+    // Check for a line comment after code.
     lead_len = get_leader_len(saved_line, &lead_flags, dir == BACKWARD, true);
     if (lead_len == 0 && do_cindent) {
       comment_start = check_linecomment(saved_line);
       if (comment_start != MAXCOL) {
         lead_len = get_leader_len(saved_line + comment_start,
-                                  &lead_flags, dir == BACKWARD, true);
+                                  &lead_flags, false, true);
         if (lead_len != 0) {
           lead_len += comment_start;
           if (did_do_comment != NULL) {

src/nvim/testdir/test_textformat.vim

@@ -238,6 +238,29 @@ func Test_format_c_comment()
   END
   call assert_equal(expected, getline(1, '$'))
 
+  " Using "o" repeates the line comment, "O" does not.
+  %del
+  let text =<< trim END
+      nop;
+      val = val;      // This is a comment
+  END
+  call setline(1, text)
+  normal 2Go
+  let expected =<< trim END
+      nop;
+      val = val;      // This is a comment
+                      //
+  END
+  call assert_equal(expected, getline(1, '$'))
+  normal 2GO
+  let expected =<< trim END
+      nop;
+
+      val = val;      // This is a comment
+                      //
+  END
+  call assert_equal(expected, getline(1, '$'))
+
   bwipe!
 endfunc