patch 8.2.4907: some users do not want a line comment always inserted

Problem:    Some users do not want a line comment always inserted.
Solution:   Add the '/' flag to 'formatoptions' to not repeat the comment
            leader after a statement when using "o".

src/testdir/test_textformat.vim

@@ -278,6 +278,18 @@ func Test_format_c_comment()
                       //
   END
   call assert_equal(expected, getline(1, '$'))
+  3delete
+
+  " No comment repeated with a slash in 'formatoptions'
+  set fo+=/
+  normal 2Gox
+  let expected =<< trim END
+      nop;
+      val = val;      // This is a comment
+      x
+  END
+  call assert_equal(expected, getline(1, '$'))
+  set fo-=/
 
   " using 'indentexpr' instead of 'cindent' does not repeat a comment
   setl nocindent indentexpr=2