Conflict detection losing base context

```sh
❱ ec --version
ec v0.3.0
❱ cat base.txt
line1
line2
❱ cat left.txt
line1
line2
left line
❱ cat right.txt
line1
line2
right line
❱ cat merged.txt
❱ ec base.txt left.txt right.txt merged.txt
❱ cat merged.txt
left line
right line
```

I selected 'b' Both, then 'w' Write, then 'q' Quit.

I expected `line1` and `line2` to be present, but they are not.