fix(prompt): "%" prefix repeated on newlines with formatoptions+=r #34584

Problem:
With `formatoptions+=r`, the prompt prefix "%" is treated as
comment-start (because of global default 'comments' option contains
"%"), so it gets added to the start of the line when a new line
is input in a prompt.

Solution:
Unset the 'comments' option in prompt buffers by default.

src/nvim/optionstr.c

@@ -696,8 +696,11 @@ const char *did_set_buftype(optset_T *args)
       || opt_strings_flags(buf->b_p_bt, opt_bt_values, NULL, false) != OK) {
     return e_invarg;
   }
-  // buftype=prompt: set the prompt start position to lastline.
+  // buftype=prompt:
   if (buf->b_p_bt[0] == 'p') {
+    // Set default value for 'comments'
+    set_option_direct(kOptComments, STATIC_CSTR_AS_OPTVAL(""), OPT_LOCAL, SID_NONE);
+    // set the prompt start position to lastline.
     pos_T next_prompt = { .lnum = buf->b_ml.ml_line_count, .col = 1, .coladd = 0 };
     RESET_FMARK(&buf->b_prompt_start, next_prompt, 0, ((fmarkv_T)INIT_FMARKV));
   }

test/functional/legacy/prompt_buffer_spec.lua

@@ -273,6 +273,22 @@ describe('prompt buffer', function()
       {1:~                        }|*3
       {5:-- INSERT --}             |
     ]])
+
+    -- % prompt is not repeated with formatoptions+=r
+    source([[
+      bwipeout!
+      set formatoptions+=r
+      set buftype=prompt
+      call prompt_setprompt(bufnr(), "% ")
+    ]])
+    feed('iline1<s-cr>line2')
+    screen:expect([[
+      other buffer             |
+      % line1                  |
+      line2^                    |
+      {1:~                        }|*6
+      {5:-- INSERT --}             |
+    ]])
   end)
 
   it('can put (p) multiline text', function()