Distinguish between regional undo and undo to the beginning in undo-equiv-table

[Yuan Fu]

Currently if I undo to the beginning of the pending-undo-list, buffer-undo-list 
is mapped to t in undo-equiv-table. Meanwhile, if the undo is undo-in-region, 
the buffer-undo-list is also mapped to t in undo-equiv-table.

That means my package that constructs an undo tree from buffer-undo-list cannot 
distinguish the two and thus cannot work incorrectly. Is it ok to map one type 
of undo to something other than t? Maybe a symbol 'undo-in-region? Would that 
break anything? Or maybe we can create another hash table to mark 
undo-in-region undos.

I can explain how does my package use undo-equiv-table if needed.

Yuan