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.

1 files changed, 1 insertions, 1 deletions

diff --git a/helix-core/src/history.rs b/helix-core/src/history.rs
index b99e969f..1aac38d9 100644
--- a/helix-core/src/history.rs
+++ b/helix-core/src/history.rs
@@ -131,7 +131,7 @@ impl History {
             .map(|&n| self.revisions[n].inversion.clone());
         let down_txns = down.iter().map(|&n| self.revisions[n].transaction.clone());
 
-        up_txns.chain(down_txns).reduce(|acc, tx| tx.compose(acc))
+        down_txns.chain(up_txns).reduce(|acc, tx| tx.compose(acc))
     }
 
     /// Undo the last edit.