This is a pure refactoring PR that goes through all the git-related APIs
exposed by the worktree crate and minimizes their use outside that
crate, migrating callers of those APIs to read from the GitStore
instead. This is to prepare for evacuating git repository state from
worktrees and making the GitStore the new source of truth.
Other drive-by changes:
- `project::git` is now `project::git_store`, for consistency with the
other project stores
- the project panel's test module has been split into its own file
Release Notes:
- N/A
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Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
