Index is left out of sync after autofixing unstaged hunks

[torbiak]

For v0.003000 a temp index is used regardless whether the hunks are staged or not, but this leaves the original index out-of-sync with any changes that git-autofixup commits. The index can be synced with the new HEAD using git restore --staged.

Thanks to @krobelus for reporting this [here](https://github.com/torbiak/git-autofixup/issues/5#issuecomment-733282696].