Closes #13304

Release Notes:

- Add global `git status` and `git diff` on/off in one place instead of
control everywhere

We can first review to ensure this change meets both `Zed` and user
requirements, as well as code rules. Currently, we only support
user-level settings. We can wait for this PR:
https://github.com/zed-industries/zed/pull/43173 to be merged, then
modify it to support both user and project levels.