## Summary

Fixes a crash I hit running Zed Preview against a checkout with many
linked git worktrees. The panic was:

```
thread '<unnamed>' panicked at crates/worktree/src/worktree.rs:5334:25:
update_git_repositories: .git path outside worktree root is not a gitfile: "/Users/eric/repo/zed/.git"
```

The `debug_assert!` in `update_git_repositories` was added in #53443 to
catch the case where a `.git` path outside the worktree root is not a
gitfile. The comment there explained that a `.git` outside the root
should always be a gitfile (as in a linked worktree or submodule), so
the assertion was meant to flag "should never happen" paths.

But there's a second legitimate case: after a linked worktree's
repository has been unregistered from `git_repositories` (for example
because its gitfile was removed, or because the filesystem watcher for
the common git dir lost sync and rescan-driven cleanup dropped the
entry), a subsequent rescan event on the main repo's `.git` directory
arrives at the linked worktree's scanner with the common git dir as the
`dot_git_dir`. That path:

- is outside the linked worktree's root (so it doesn't strip-prefix
cleanly), and
- is a real directory (not a gitfile), because it's the main repo's
`.git`.

So the assertion fires, but `continue` is already the right thing to do
— there's simply nothing left for this scanner to do with a path that
isn't its repository anymore.

On macOS, the trigger in practice is the FSEvents API setting
`MustScanSubDirs` / `UserDropped` / `KernelDropped` on an event (which
`notify` surfaces as `need_rescan()`), which our `FsWatcher` converts
into a `PathEvent { path: <watched path>, kind: Rescan }`. Because every
linked worktree registers a watcher on the same shared common git dir,
one kernel drop fans out into many rescan callbacks, and any one of them
hitting a worktree whose repo was just unregistered triggers the panic.

## Changes

- `crates/worktree/src/worktree.rs` — drop the `debug_assert!`, broaden
the comment to cover both cases.
- `crates/worktree/tests/integration/main.rs` — add a failing regression
test that drives the exact sequence (repo unregistered, then a Rescan
event on the common git dir) and asserts it doesn't panic.

The two commits are split so the test commit reproduces the panic on its
own, and the fix commit on top makes it pass.

Release Notes:

- N/A