vim-patch:8.2.4907: some users do not want a line comment always inserted (#18463)

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".
2bf875f881

src/nvim/change.c

@@ -1186,7 +1186,8 @@ int open_line(int dir, int flags, int second_line_indent, bool *did_do_comment)
   end_comment_pending = NUL;
   if (flags & OPENLINE_DO_COM) {
     lead_len = get_leader_len(saved_line, &lead_flags, dir == BACKWARD, true);
-    if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD) {
+    if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD
+        && !has_format_option(FO_NO_OPEN_COMS)) {
       // Check for a line comment after code.
       comment_start = check_linecomment(saved_line);
       if (comment_start != MAXCOL) {

src/nvim/option_defs.h

@@ -67,6 +67,7 @@
 #define FO_WRAP_COMS    'c'
 #define FO_RET_COMS     'r'
 #define FO_OPEN_COMS    'o'
+#define FO_NO_OPEN_COMS '/'
 #define FO_Q_COMS       'q'
 #define FO_Q_NUMBER     'n'
 #define FO_Q_SECOND     '2'
@@ -85,7 +86,7 @@
 
 #define DFLT_FO_VI      "vt"
 #define DFLT_FO_VIM     "tcqj"
-#define FO_ALL          "tcroq2vlb1mMBn,aw]jp"   // for do_set()
+#define FO_ALL          "tcro/q2vlb1mMBn,aw]jp"   // for do_set()
 
 // characters for the p_cpo option:
 #define CPO_ALTREAD     'a'     // ":read" sets alternate file name
@@ -118,7 +119,7 @@
 #define CPO_REMMARK     'R'     // remove marks when filtering
 #define CPO_BUFOPT      's'
 #define CPO_BUFOPTGLOB  'S'
-#define CPO_TAGPAT      't'
+#define CPO_TAGPAT      't'     // tag pattern is used for "n"
 #define CPO_UNDO        'u'     // "u" undoes itself
 #define CPO_BACKSPACE   'v'     // "v" keep deleted text
 #define CPO_FWRITE      'W'     // "w!" doesn't overwrite readonly files

src/nvim/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