Fix transaction composition order in History::changes_since (#4981)

* Add a undo/redo split test case for crossing branches

* history: Switch up/down transaction chaining order

The old code tends to work in practice because, usually, either up_txns
or down_txns are empty. When both have contents though, we can run into
a panic trying to compose them all since they will disagree on the
length of the text. This fixes the panic test case in the parent
commit.

0 files changed, 0 insertions, 0 deletions