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 #9426)

src/change.c

@@ -1655,13 +1655,14 @@ open_line(
 	lead_len = get_leader_len(saved_line, &lead_flags,
 							dir == BACKWARD, TRUE);
 #ifdef FEAT_CINDENT
-	if (lead_len == 0 && do_cindent)
+	if (lead_len == 0 && do_cindent && dir == FORWARD)
 	{
+	    // Check for a line comment after code.
 	    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;

src/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
 

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3934,
 /**/
     3933,
 /**/