fix(prompt): prompt_setprompt sets cursor col unnecessarily

Problem: prompt_setprompt adjusts the cursor's column number even when it's not
on the prompt's line.

Solution: only adjust when on the prompt's line.

src/nvim/eval/buffer.c

@@ -800,7 +800,7 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
       cursor_col += new_prompt_len - old_prompt_len;
     }
 
-    if (curwin->w_buffer == buf) {
+    if (curwin->w_buffer == buf && curwin->w_cursor.lnum == prompt_lno) {
       coladvance(curwin, cursor_col);
     }
     changed_lines_redraw_buf(buf, prompt_lno, prompt_lno + 1, 0);

test/functional/legacy/prompt_buffer_spec.lua

@@ -858,7 +858,24 @@ describe('prompt buffer', function()
     ]])
     eq({ 1, 13 }, api.nvim_buf_get_mark(0, ':'))
 
+    -- Cursor not moved when not on the prompt line.
+    feed('<CR>user input<Esc>k')
+    screen:expect([[
+      new-prompt > user inpu^t  |
+      new-prompt > user input  |
+      {1:~                        }|*7
+                               |
+    ]])
+    set_prompt('<>< ')
+    screen:expect([[
+      new-prompt > user inpu^t  |
+      <>< user input           |
+      {1:~                        }|*7
+                               |
+    ]])
+
     -- No crash when setting shorter prompt than curbuf's in other buffer.
+    feed('i<C-O>zt')
     command('new | setlocal buftype=prompt')
     set_prompt('looooooooooooooooooooooooooooooooooooooooooooong > ', '') -- curbuf
     set_prompt('foo > ')