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/edit.c b/src/edit.c
index 73e8337..934f6c1 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -3946,8 +3946,11 @@
 #endif
 		((curwin->w_cursor.lnum == 1 && curwin->w_cursor.col == 0)
 		    || (!can_bs(BS_START)
-			&& (arrow_used
-			    || (curwin->w_cursor.lnum == Insstart_orig.lnum
+			&& ((arrow_used
+#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)