Map redo records for undo in region to 'undo-in-region

Map redo records for undo in region to 'undo-in-region

* lisp/simple.el (undo-equiv-table): Add explaination for
undo-in-region, undo to the beginning of undo list and null undo.
(undo): If equiv is 'undo-in-region, empty or t, set pending-undo-list
to t.  If the redo is undo-in-region, map buffer-undo-list to
'undo-in-region instead of t, if it is an identity mapping, map to
'empty.
(undo-make-selective-list): Only continue when ulist is a proper list.
* test/lisp/simple-tests.el (simple-tests--undo): Add test for
undo-only in region.
(simple-tests--sans-leading-nil): New helper function.
(simple-tests--undo-equiv-table): New test for 'undo-equiv-table'.