Make undo/redo work for:
authornick_m <mainsbridge@gmail.com>
Fri, 6 Feb 2015 10:54:27 +0000 (21:54 +1100)
committernick_m <mainsbridge@gmail.com>
Fri, 6 Feb 2015 10:54:27 +0000 (21:54 +1100)
commit6b9415aedb35d03561ef16d846f08bc0fc7c57c7
treea00c8a23c9528451c8e5d2ffedd761074335174c
parentec37c835074e0218b1cf216d4099e18fa77dcdbc
Make undo/redo work for:
undo (n) where n > 1
redo (m) where m < n
new transaction.

Previously the redo list was left untouched.
This would lead to utter nonsense in the redo list.

AFAICT this never worked.
libs/pbd/undo.cc