Adds basic support for Git submodules

[eamodio]

Improves the handling of Git submodules by treating them as first-class repository entities alongside worktrees. This update ensures that submodules (that are discovered by VS Code) are correctly identified and provides better visual differentiation within the user interface.

Key Enhancements

• Submodule Discovery: Enhances repository tracking by identifying nested submodules using rev-parse and tracking parent repository relationships.
• Improved UI/UX: Distinguishes submodules in the repository tree and quick picks with specific icons and tooltips.
• Contextual Menu Control: Updates menu "when" clauses to filter out submodule-specific files from certain actions where they are not yet supported, such as file-level sharing or deep linking.
• Diff & Navigation: Adds logic to handle 160000 file modes, allowing for better navigation and diffing of subproject commits.

Closes #1048, #1803

[eamodio]

There are many caveats and gaps here — this gets some basic submodule support to largely align with VS Code's support.

[augmentcode]

🤖 Augment PR Summary

Summary: Adds first-class support for Git submodules alongside worktrees, improving discovery, UI differentiation, and diff/navigation behavior.

Key Changes:

• Enhances repo discovery via a single rev-parse call that also captures a submodule’s superproject working tree.
• Extends revision URIs with submoduleSha metadata and updates best-revision resolution to accept Uri inputs.
• Updates diff commands to detect submodule working SHAs and generate appropriate diff URIs/titles.
• Improves git log parsing by switching to --raw to capture file modes (e.g., 160000) for submodule detection.
• Adjusts filesystem provider behavior to render submodule entries as Subproject commit <sha> and to treat gitlinks as files.
• Adds status parsing/propagation of submodule indicators and threads mode/submodule info through file change models.
• Updates views, icons, tooltips, and context values to visually distinguish submodules.
• Restricts unsupported actions via menu when clauses for submodule file items.
• Adds telemetry for opened submodule counts and a dedicated log-parser test suite covering mode/submodule scenarios.

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[augmentcode]

Review completed. 3 suggestions posted.

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