Panic in built-in diff editor when replacing directory with file

[arxanas]

I'm not sure if I should make a new issue, but I got a consistent unreachable panic corner case type of thing that seems related to this (there is a possibility this is precisely what you guys are trying to fix 😅)

Basically if you replace a folder with a file (in my case it was a symlink) and then try to split you get a panic.

minimal repro:

λ jj git init test
Initialized repo in "test"
λ cd test
λ mkdir folder
λ touch folder/.keep
λ jj ci -m 'added folder'
Working copy now at: (x) c76fc22 (empty) (no description set)
Parent commit      : (u) 9092cba added folder
λ rm -r folder
λ touch folder
λ jj split
Hint: Using default editor ':builtin'; run `jj config set --user ui.diff-editor :builtin` to disabl
e this message.
thread 'main' panicked at cli/src/merge_tools/builtin.rs:205:13:
internal error: entered unreachable code: list of changed files included a tree: TreeId("29a422c192
51aeaeb907175e9b3219a9bed6c616")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Originally posted by @necauqua in #3702 (comment)

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On this topic, I've successfully used property-based testing for incremental file changes in a similar context. It would be good to try it here to fuzz for panics, at least.