patch 8.2.0839: dropping modifier when putting a character back in typeahead

Problem:    Dropping modifier when putting a character back in typeahead.
Solution:   Add modifier to ins_char_typebuf(). (closes #6158)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index e076241..b74ad2c 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -2,6 +2,7 @@
 
 source shared.vim
 source term_util.vim
+source view_util.vim
 
 func Test_messages()
   let oldmore = &more
@@ -305,4 +306,12 @@
   endif
 endfunc
 
+func Test_mapping_at_hit_return_prompt()
+  nnoremap <C-B> :echo "hit ctrl-b"<CR>
+  call feedkeys(":ls\<CR>", "xt")
+  call feedkeys("\<C-B>", "xt")
+  call assert_match('hit ctrl-b', Screenline(&lines - 1))
+  nunmap <C-B>
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab