Render submodule bumps as commit-pointer diffs

[Copilot]

Submodule changes currently open checkout directories or request nonexistent blob content, causing failures in the PR tree and Changes view.

• Pointer rendering: Recognize canonical gitlink patches and display old/new pointers through virtual pr: documents:

  -Subproject commit <old-sha>
  +Subproject commit <new-sha>

• Consistent navigation: Route submodule bumps, additions, removals, and renames with pointer changes to diffs instead of filesystem paths.
• Refresh safety: Embed pointer content in document URIs so cached PR providers cannot substitute stale content after another push.

Ordinary file handling remains unchanged.

• Fixes Opening a submodule bump in the PR changes view fails: "Unable to read file ... that is actually a directory" #8981

[azure-pipelines]

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