patch 8.1.1055: CTRL-G U in Insert mode doesn't work for shift-Left

Problem:    CTRL-G U in Insert mode doesn't work to avoid splitting the undo
            sequence for shift-left and shift-right.
Solution:   Also check dont_sync_undo for shifted cursor keys. (Christian
            Brabandt)

src/testdir/test_mapping.vim

@@ -140,9 +140,32 @@ func Test_map_cursor()
   imapclear
 endfunc
 
+func Test_map_cursor_ctrl_gU()
+  " <c-g>U<cursor> works only within a single line
+  nnoremap c<* *Ncgn<C-r>"<C-G>U<S-Left>
+  call setline(1, ['foo', 'foobar', '', 'foo'])
+  call cursor(1,2)
+  call feedkeys("c<*PREFIX\<esc>.", 'xt')
+  call assert_equal(['PREFIXfoo', 'foobar', '', 'PREFIXfoo'], getline(1,'$'))
+  " break undo manually
+  set ul=1000
+  exe ":norm! uu"
+  call assert_equal(['foo', 'foobar', '', 'foo'], getline(1,'$'))
+
+  " Test that it does not work if the cursor moves to the previous line
+  " 2 times <S-Left> move to the previous line
+  nnoremap c<* *Ncgn<C-r>"<C-G>U<S-Left><C-G>U<S-Left>
+  call setline(1, ['', ' foo', 'foobar', '', 'foo'])
+  call cursor(2,3)
+  call feedkeys("c<*PREFIX\<esc>.", 'xt')
+  call assert_equal(['PREFIXPREFIX', ' foo', 'foobar', '', 'foo'], getline(1,'$'))
+  nmapclear
+endfunc
+
+
 " This isn't actually testing a mapping, but similar use of CTRL-G U as above.
 func Test_break_undo()
-  :set whichwrap=<,>,[,]
+  set whichwrap=<,>,[,]
   call feedkeys("G4o2k", "xt")
   exe ":norm! iTest3: text with a (parenthesis here\<C-G>U\<Right>new line here\<esc>\<up>\<up>."
   call assert_equal('new line here', getline(line('$') - 3))