[BUG REPORT] `--autostash` rebase + pre-rebase error ends in corrupt state

If you perform a rebase using `--autostash`, and the pre-rebase hook exits with a non-zero exit code, your worktree is left in some sort of invalid state. This is reproducible with the following script:

    git init --initial-branch main
    echo -e '#!/bin/sh\nexit 1' > .git/hooks/pre-rebase
    chmod +x .git/hooks/pre-rebase
    
    git commit --allow-empty -m "initial commit"
    
    git switch -c feature1 main
    
    touch a.txt
    git add a.txt
    git commit -m "add a"
    
    git switch -c feature2 main
    
    touch b.txt
    git add b.txt
    git commit -m "add b"
    
    echo "content" > b.txt
    git rebase --autostash feature1

At this point, `git status` indicates that you are rebasing and `__git_ps1` also shows simply `(|REBASE)`. However, attempting to `git rebase --abort` or `git rebase --continue` gives "warning: could not read '.git/rebase-merge/head-name': No such file or directory", and does not appear to change the worktree state.

You're able to recover from this by using `git rebase --quit` and then resetting to the commit you were on before attempting the rebase.

This issue only occurs if there is actually some modification to stash; the presence of `--autostash` by itself does not trigger it.

If the above script is adjusted to make no uncommitted modifications, and rebase without using `--autostash`, then the rebase is simply aborted as expected when the pre-rebase hook fails.

I've reproduced this in the following OS/git version combinations:

- Gentoo Linux, git v2.46.0
- MacOS Sonoma 14.5, git v2.46.0
- Windows 11, git 2.44.0.windows.1