Prevent multibuffer items from switching active repository

[mengh04]

Fixes the git panel's active repository being silently overridden by the cursor position in ProjectDiff.

Summary

When a multi-buffer item (e.g. ProjectDiff) gains focus, the workspace calls set_active_repo_for_path with the cursor's project path to automatically switch the active git repository. This was safe when multi-buffer items returned None from active_project_path, but PR #57236 changed ProjectDiff to return the cursor-based path (for project panel highlighting), inadvertently allowing the cursor position to override the user's explicit repository selection.

Fix the issue by restoring the Singleton guard that was removed in PR #57236: only call set_active_repo_for_path when the active item is backed by a single buffer.

Additionally, subscribe ProjectDiff to ActiveRepositoryChanged events so that its branch_diff updates when the user switches repos via the repository selector.

Changes

• workspace.rs: Only call set_active_repo_for_path when the active item's buffer_kind is Singleton. Multi-buffer items (e.g. ProjectDiff, diagnostics, search results) span multiple repositories, so their cursor position should not trigger an automatic repository switch.
• project_diff.rs: Subscribe to GitStoreEvent::ActiveRepositoryChanged to keep branch_diff in sync when the active repository is changed via the repository selector.

Before

before.mp4

After

after.mp4

Closes #58792

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 git panel repository being silently overridden when clicking files in a multi-repository workspace.

[mengh04]

I've just added a regression test covering this fix.