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)

src/edit.c

@@ -3946,8 +3946,11 @@ ins_bs(
 #endif
 		((curwin->w_cursor.lnum == 1 && curwin->w_cursor.col == 0)
 		    || (!can_bs(BS_START)
-			&& (arrow_used
+			&& ((arrow_used
-			    || (curwin->w_cursor.lnum == Insstart_orig.lnum
+#ifdef FEAT_JOB_CHANNEL
+				&& !bt_prompt(curbuf)
+#endif
+			) || (curwin->w_cursor.lnum == Insstart_orig.lnum
 				&& curwin->w_cursor.col <= Insstart_orig.col)))
 		    || (!can_bs(BS_INDENT) && !arrow_used && ai_col > 0
 					 && curwin->w_cursor.col <= ai_col)

src/testdir/test_prompt_buffer.vim

@@ -122,6 +122,14 @@ func Test_prompt_garbage_collect()
   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

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1976,
 /**/
     1975,
 /**/