git: Fix a race that caused incorrect hunk staging information

[cole-miller]

In GitStore::open_diff_internal, when opening a DiffKind::Unstaged diff, we previously always overwrote the unstaged_diff field of BufferGitState. This causes a bug in the following situation:

• We call open_unstaged_diff, and it hits an await point where it is loading the index text
• While that task is suspended, we call open_uncommitted_diff. It reaches open_diff_internal and sets buffer_git_state.uncommitted_diff and buffer_git_state.unstaged_diff. It also sets the secondary diff for the uncommitted diff to be the unstaged diff that it just opened.
• The open_unstaged_diff task wakes up and enters open_diff_internal. It creates a new entity for the unstaged diff and overwrites buffer_git_state.unstaged_diff with it, but it doesn't update the secondary diff of the uncommitted diff, which still has the entity that was produced in the previous step.
• Now the uncommitted diff's secondary diff will never receive updates from the GitStore, causing staging information to be incorrect.

The fix is for open_diff_internal to not overwrite an existing unstaged diff.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed a bug that could cause diff hunks to have an incorrect staged status.