patch 8.2.1976: cannot backspace in prompt buffer after using cursor-left

Problem:    Cannot backspace in prompt buffer after using cursor-left. (Maxim
            Kim)
Solution:   Ignore "arrow_used" in a prompt buffer. (closes #7281)
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index 8c266eb..00b97f8 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -122,6 +122,14 @@
   bwipe!
 endfunc
 
+func Test_prompt_backspace()
+  new
+  set buftype=prompt
+  call feedkeys("A123456\<Left>\<BS>\<Esc>", 'xt')
+  call assert_equal('% 12346', getline(1))
+  bwipe!
+endfunc
+
 " Test for editing the prompt buffer
 func Test_prompt_buffer_edit()
   new