Updates #44089
- Restores the ability to have a partially staged/`Indeterminate` status
for the git panel checkboxes
- Removes the `optimistic_staging` logic, since its stated purpose is
served by the `PendingOps` system in the `GitStore` (which may have
bugs, but we should fix them in the git store rather than adding another
layer)
Release Notes:
- Fixed partially-staged files not being represented accurately in the
git panel.
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Co-authored-by: Anthony Eid <hello@anthonyeid.me>