When a worktree is created, we walk up the ancestors of the root path
trying to find a git repository. In particular, if your `$HOME` is a git
repository and you open some subdirectory of `$HOME` that's *not* a git
repository, we end up scanning `$HOME` and everything under it looking
for changed and untracked files, which is often pretty slow. Consistency
here is not very useful and leads to a bad experience.

This PR adds a special case to not consider `$HOME` as a containing git
repository, unless you ask for it by doing the equivalent of `zed ~`.

Release Notes:

- Changed the behavior of git features to not treat `$HOME` as a git
repository unless opened directly