Log tab panics and aborts the process when the cwd has been deleted (gix discovery reads the process cwd)

## Summary

Any operation that leaves the process cwd deleted makes the unfiltered Log tab panic, and the panic aborts the whole process rather than just the log thread. The most reachable way in is stashing untracked files from inside a subdirectory, since libgit2 removes that directory as part of the stash.

```
thread panicked at asyncgit/src/revlog.rs:186:14:
failed to fetch: Gix(Discover(Discover(CurrentDir(Os { code: 2, kind: NotFound, message: "No such file or directory" }))))
Rayon: detected unexpected panic; aborting
```

## Why the repo path cannot fix it

`gix` discovery reads the process cwd unconditionally, before it looks at the path it was given:

- `gix/src/discover.rs:45` in `discover_opts`: `options.current_dir = Some(gix_fs::current_dir(false).map_err(upwards::Error::CurrentDir)?);`
- `gix-fs/src/lib.rs:56`: `let cwd = std::env::current_dir()?;`

So passing an absolute repository root does not help. I reproduced it with an explicit absolute path, and again with `RepoPath::resolve_root()` applied exactly as `src/main.rs` does at startup. Identical panic both times.

The entry point is `gix_repo()` at `asyncgit/src/sync/repository.rs:89`, reached from `AsyncLog::fetch_helper_without_filter` at `asyncgit/src/revlog.rs:266`, whose error is `.expect("failed to fetch")` at `revlog.rs:186`.

## Why git2 paths survive

`repo()` at `asyncgit/src/sync/repository.rs:82` uses `git2::Repository::open_ext`, which does not consult the cwd for an absolute path. That is why `get_stashes` and the filtered log keep working while the unfiltered log dies. In other words this only fires when no commit filter is active.

## Reproduction

A scratch crate linking `asyncgit`: create a repo, add an untracked subdirectory, set the process cwd inside it, `stash_save(..., untracked=true)`, then call `AsyncLog::fetch`.

```
[after stash] cwd=Err(Os { code: 2, kind: NotFound })
[git2 get_stashes] OK, 1 stashes
[gix discover] ERR on "/tmp/.../scratch": Discover(CurrentDir(Os { code: 2, kind: NotFound }))
panicked at asyncgit/src/revlog.rs:186: failed to fetch: Gix(Discover(...))
```

## Possible directions

Either set the process cwd to the repository root at startup, or build `discover_opts` with `upwards::Options::current_dir` set explicitly rather than letting gix read the environment. Separately, `revlog.rs:186` turning a fetch error into `.expect` is what escalates this from a broken tab to an aborted process, so that is worth handling regardless of the cwd question.

Filed at the request of a reviewer on #3015, which fixes the git2 half of the same scenario. This one is independent of that change.

Read asyncgit/src/revlog.rs around lines 186 and 266, then asyncgit/src/sync/repository.rs around lines 82 and 89. Reproduce the deleted-cwd scenario described in the issue and trace the gix discovery failure; compare it with the git2 path that survives. Done means the unfiltered Log tab no longer aborts the process when the cwd is deleted, with the fetch failure handled without panic.

Written by the indexing model from the issue text.